You need to get very familiar with a few of the addons that do a lot for you, your unit frames addon and WeakAuras are the main ones. For unit frames I've used Stuf for a long time because it is so flexible and so fast to change things. When you're learning you don't want to get bogged down in configuring things, so the faster it is the more your creativity can flow. So if you have an idea like "I wonder if this would be better if my focus bar was huge", then you can have it done in 5 or 10 minutes, and be in a BG testing it out. etc.
It's also a good idea to have a different UI for each character/alt that you have. Make them all as different as possible. That way you will really get a feel for everything you can do before you decide what sort of thing you really like best. If you commit to one UI you can end up getting stuck in a rut of your own making, and not wanting to change much because of having put so much work into it already. If you see what I mean.
It's also good to collect bits of UI (not only WoW UIs, other games too) that you think are great as inspiration. Starting a Pinterest board is a pretty easy way to do that.