My lock was my first char and I admit I wasnt overly sure what I was doing as I went full destro for leveling and finding I was out of mana alot and just thought that was normal.
Apparently for leveling its better to go afflic at the lower levels because you have less down time between attacking (you dont run out of mana as much) I'd say go demo once you can get felguard. I leveled from 70-80 as demo and could solo so much and leveled pretty fast. Unfortunetly as I said earlier I did it wrong myself at the lower levels so could be a bit wrong since a lot has changed in the past 3 years.
In terms of professions you want, it would depend on what you wanted to do with him? If he is going to be your main and you want to be a hardcore raider then I would suggest JC/Ench for all the extra spell damage they give. If you dont already have a main to farm/pay for mats for you then JC is going to be hard. Maybe go mining/herb until lvl 58-60. You can sell the mats and make loads of gold and if you are sensible then you can keep alot of the mats required for JC along the way. Also keep alot of the Bop gear in the bank and give and boe gear to an alt for DE when you become an Enchanter.
imo just grind grind grind. it's kinda boring but it beats the hell out of the shitty quests (until you get to outland, anyway). get some music and droogz then go nuts. affliction (maybe try felguard spec at 50, a respec is cheap the first few times)
I did affliction till Felguard then Demo to 80 (well at 70 I respecced for TBC but back to Demo for lvling again :P). Affli is nice and fast then Demo was great since you could solo everything and I was never out of mana (pet tanking lifetap drain life if low mana).
I think the best spec from 71-80 is by far the FG/SL spec. It's amazing throughput with absolutely minimal downtime. I've been limited by the number of mobs around me at all times (it feels amazing to run into an area of 7-9 mobs, kill them all and more and end up with half health and mana). Here's some information on the spec: