wait til christmas and save up until then.
in the mean time, lurk the **** out of ssfg and mechanics subforums. I mean that as in read everything pertaining to maintenance and parts. find out what's good and what's not. find out how to maintain what's good to keep it good.
depending on how confident you feel by christmas, get your bike from the internet or from craigslist. if going used on craigslist, use your insane gained knowledge to make sure all the bearings are in good order, the chain and drivetrain aren't worn out, the wheels are true and tensioned etc. Also being learned will help you avoid getting ripped off.
if buying off the internet, you will know how to fix whatever breaks.
no matter the scheme, set aside about a benjamin for tools. stuff like allen wrenches and open end wrenches or ratchets you can pick up from harbor freight to save, but you'll need some bike specific tools. off the top of my head i can think of bottom bracket tool, chainring bolt wrench, lockring pliers (if you don't feel comfortable modifying a pair of slipjoints with a dremel), and a floor pump.
advantages of buying from a bike store would be that they can help pick your size and do maintenance for you. you will now do your own maintenance so you're okay there. for sizing, see if you can borrow any friends' bikes. adjust the seat for your height (heel sits on pedal when leg is fully straight so there's a slight bend in the knee when pedaling with the balls of your feet [proper foot placement]). if it doesn't go low enough, you'll need smaller. size is measured by the length of the seat tube. I'd guess for your height of 5'4" a 52-53 would be good. I'm about 5'8" and 55 works nicely for me (though i'm building up a 53.5...)
that's all i can think about right now