Now this is how I did it. It doesn't mean it's right for everyone, it's just my preference. If you have a different way, cool. Feel free to go ahead and do that. I won't hold it against you, and I will congratulate you on making the money. For me I found that this was absolutely the quickest way to get that first 100k out of the way. It actually makes more if you stick with it, but at 20 feel free to drop some of the skills and pick up something else.
That said, let's get into the meat of it.
1. The first rule on making money is, don't spend it. You can spend your money on skills, but ONLY skills. There is absolutely no reason to ever spend money on anything else. Nope, none. Anything you need can be obtained in another way. Usually through commendations. You will obtain an orange chest piece really early on in your capital city if you are Republic. Use this, and simply buy mods to upgrade it.
2. If you have a choice in commendation, or gear for rewards pick the commendation unless the reward is blue(and a big upgrade)/orange. Always take the commendation otherwise, because gear is overly abundant. It is impossible to level through your starting city, and not get a good set even if you take the commendation every chance you get. You will be able to buy gear, mods for yourself. So you don't need to bother spending money.
3. Don't bother auctioning greens, right now blues are dirt cheap. You will make more just using the vendor for your greens. I know, it looks like it might sell. The thing is that there are blues on the GTN for a few hundred. You will make more selling that green, or using it yourself/or on a companion.
Now that those three tips are out of the way, you will need a few things to really make money. You will have 3 crew skills, none of them are mission skills. The other two are for gathering. Now, you might ask me "hey doesn't not being able to craft gimp me?" No, in fact taking a crafting skill hurts you far more than it will ever help you. The only time crafting is better is at level 50 and even then it's only under certain conditions. The exception to this is Biochem because of the reusable medpacs, stims, and adrenals. Cybertech is possibly the second one because of the ability to make mods, but there is no need because the mods you can buy with commendations are actually better than the ones that can be crafted. In addition mod vendors have mods that cybertech can not make. This used to not be the case. At one point cybertech was very important so that people could make their tanking mods, but that was switched to vendors.
So what three crew skills do I suggest?
Scavenging to begin with. You will find lots of nodes to scavenge in the wild. It also allows you to harvest some mechanical things you kill. In addition it allows your group certain areas inside instances that you couldn't get to otherwise (shortcuts etc) You will not regret this, and when you get to the point that you can farm certain areas this will be your big money maker.
Example: Bronzium is a common grade 2 scavenged metal. Yet it is required by so much (armortech/armstech/cybertech) that it is in constant demand. Combine that with the fact that it's often in demand early on your main planet, but you can't farm it till the planet after your main (usually). This creates a big demand for it. As of right now it is selling for 700 per bronzium. Once you get to Taris for republic you can farm it easily. I will spend about an hour, come back with 40 to 50. Sell it 2 at a time for 1k each. Come back later, collect my money and be on my way.
Useful places to farm
-the works on Cours for Republic, at the end is an area where the robots respawn very quickly. Each one will have grade 2 metals. You can farm very quickly there. They also have some good loot, but mostly you are going to farm the metal.
-Taris all over, just ride around on your speeder and collect as you go. There is plenty of metal.
Bioanal by itself scavenging will make you lots of money. The bonus comes in when you mix that with bio. Being able to scan and get items from that is huge. The green goo won't sell, you can just vendor it or send it to an alt. Once you get to taris though you begin to get parts that will sell really well. Scan everything. This also has the benefit of giving you certain advantages in instances. Mostly though it means that Taris, alderan, tat are gold mines for you.
The third is where most people would expect slicing, well I have news for you. Slicing didn't make as much as the two above before the nerf, and it damn sure won't now. Slicing is a waste, and it only looks good on paper because most people didn't know how to make money yet. The people like me that were out there sitting at couple hundred k early on already knew slicing was a waste. No, for a third we are going to go with archeology. Wait, what? I know you are kind of wondering what the hell I'm talking about suggesting archeology over slicing. Well, archeology is one of those sleeper gathering skills. Most people don't know about it, the ones that do are cleaning house with it. You will make (on average) more money off archeology than the two above combined. Why? It's simple, people want different color crystals. Artifice has to make them, but for that they need color crystals. Also take into account that you are providing synthweaving/artifice with what they need to craft. You will make a butt load of credits just off selling Lost ARtifact Fragments. Those sell for min 1k right now, you can get quiet a few from 1 node.
http://www.torhead.com/item/chpgDN1
Go take a look at everything that requires that one item. You can farm them so easy also, you just have to know where to do it. On Taris there are a few spots where these spawn very quickly. In fact if you make a circle you will be collecting nonstop with scavenging/biochem/archeology. You won't ever have to wait on respawn. The circle starts near the Hospital, extends east then south twoards the Endar Spire, then circles back around. You will be collecting the entire time. There is actually a fleet shuttle at the town there so you won't need to walk far to put stuff on the GTN. Then go back. You circle will net you 50k to 80k an hour (my experience). There is very little combat here, but if you want you can adjust this. There is another circle like this one that passes around X elite name here. You can farm the elite quite easily for gear/commendations, and you will be able to farm up lots of metal. I didn't suggest it before because the amount of mobs can be irritating if you don't have a speeder. Considering you can scan many of them with either bio, or scavenging though it could be a bonus.
Now to address the inevitable
-but I can make money with slicing without doing anything, well so can I. I can send my guys out on an artifice mission and make twice what you make on slicing. Actually if it's rich I could make over tripple.
-your way is all based on other players buying your stuff, what if they don't? LOL, they will. BW has ensured they will because they are actually saving money by buying it directly from me. The system is set up so that a person will slowly go broke if they try to craft in a traditional manner. They can not continue to send their guys out, it just won't work. With the current system being that you don't actually ever have to buy anything from another player, the people crafting will never gain anything. They will have to buy from you. It's not like they are going to give up the 10s/100s of thousands they have thrown away. Nope, they will buy and keep buying. Don't ever worry you will not have people to buy from you. The game is growing quickly, and there is a new sucker born every few seconds.
-what stops the market from being over crowded? Nothing, and it is on some things. The problem is that bioware has made a select few materials required in huge amounts across multiple crafting professions. Those are the ones you want to get. Do not bother with the others, target the ones that are required by multiple professions.
alum, bronzium, any artifacts, ignore rubat, all color crystals, ignore green goo, anything else from bio can be useful, chanlon, plasteel, vendor desh, laminoid is a huge one*
Laminoid is a special case, because it's required for so much early on. Yet there are very few ways to actually get it early on. It's not usual for me to sell laminoid for 1500 per, and people WILL pay it. They don't have a choice until the current bugs are fixed (might have been this patch, haven't checked) until then though squeeze the blood out of that stone.
That's the area you want to start with. There are two rare elites that spawn here, very easy to kill both drop good loot. Also good money in this area too for the mobs that get in your way. This is where the first metal spawns, you will grab that. Then move in a circle along the road coming back. There are also a few chests that spawn along the route, giving you more money/items to sell auction. Remember all greens go to the vendor, blues it's your call.
Doing the above you will make your first 100k easily by level 20. My trooper in the pic is level 25, I have riding already, mount and I have almost 100k set aside after getting riding for my other two characters. This is with buying gear as you can probably tell from looking at him, broke my own rule but once you get that first 100k go ahead and enjoy some of your credits.
I know there are other ways, I know you can do this without putting any effort into it. This is just my preferred way. Feel free to post your own tips, I don't mind at all. Just don't post stupid stuff like "hurhurr do slicing" because that actually takes longer.
This is the elite
http://www.torhead.com/npc/4uqLCZO
and a screen showing her and the loot I got from her
As you can see her respawn is about 6 minutes, so she will be up to farm every time you come back around.
For anyone interested in trying it I'm going to include a map of the route I usually take. Keep in mind this is much easier once you get your speeder.