Yep, the pinned topics here help a bunch, also reading other people's journals about their tanks helped me getting started, and always ask lots of questions no matter how dumb they may seem.
Between a 15 and 20g, i'd get the 20g just because it is bigger and will therefore be easier to keep levels where they're suppose to be. You won't need a skimmer because the tanks won't be too big, I have a 29g and have never had a reason to buy one yet.
I personally, instead of spending hundreds on live rock, bought smaller pieces called "rubble rock" which had been curing in my lfs for around 2 months, looked wonderful and only cost $2 a pound
. I probably bought around 30-40 pieces which totaled 20pounds and then glued them together with epoxy, and was able to create caves and other cool looking things.
So if you can find that, that could be a money saver.
For sand, I definitely recommend Aragamax sand, no matter what else someone may suggest to you. And i wouldn't waste the extra money on live sand (which is the sand that is sold in a bag full of water), its not worth it, and your sand will turn live anyways after a period.