ferrouslepidoptera has good advice up there, but as he says, there's always more to be said.

First, I'd advise you in the strongest possible terms to avoid any kind of vibrato device or "whammy bar" as people seem to call them. ferrouslepidoptera understates how wretched they can be. They require delicate adjustment. You probably don't have money to waste on that. Any beginner-priced guitar you buy with a vibrato tailpiece is very likely to be a piece of shit to begin with, and if it's used it'll be out of whack to boot. What I mean to say is that in many cases you'll be lucky to keep it in tune for three minutes at a stretch, and your life will be a constant misery with it. Eddie van Halen has very nice guitars, and professional guitar techs to look after them; you don't. For a beginner, in practical terms, the thing is a useless gimmick anyway. You'd be better off spending your first year or so learning to play the instrument. You'll have enough to contend with doing just that. A fixed bridge can't get out of whack, so you'll be able to concentrate on making music instead of ad hoc mechanical engineering. A cheap Floyd Rose in the hands of a naïve beginner is a one-way ticket to Hell; don't ask me how I know.

In fact, avoid anything gimmicky. Simplicity in all things; why pay extra for more problems? There's less to go wrong, and you're not yet in any shape to be fixing the thing. Try not to worry too much about how cool it looks. When you're fighting with a cool-looking guitar that won't stay in tune or sounds like shit, you'll wish you had an ugly one that behaved itself. This is counter-intuitive, but think of it as a musical instrument first, and a fashion accessory second. If you want both, it'll cost you.

Second: The density of the guitar is important. Density == sustain. Sustain matters a lot. All other things being equal, go for the heavier guitar.

Third: ferrouslepidoptera advises you to bring a knowledgeable friend. He is a wise man. Everybody who wants to sell you a guitar is a thief. They are fiends in human form. They will drink your blood. When you walk into a music store with a baffled look on your face and money in your pocket, the salesman's eyes will light up with little dollar signs: SUCKERRR! Two out of three cheap used guitars are worthless junk. He's got one with your name on it. When your friend starts muttering about how the frets are worn, the salesman will probably become hostile (that's if the frets are worn; if not, your friend has made an ass of himself). If the salesman becomes hostile, you're in good shape. That hostility is a sign that he's had to resign himself to making an honest living that day. It means you've got a fighting chance.

There is a food chain here, and you're on the wrong end of it. You will probably get ripped off, whatever you do. A year or so later, you'll unload that hunk of junk on some bright-eyed sucker who thinks it looks cool, and in that moment you will be enlightened.

Finally: Humbuckers are indeed louder than single-coil pickups and they tend to have less hum (hence the name), but that's not the whole story: They also sound thicker and punchier.