i agree that a pawn shop is not a good idea - it's either stolen or junk...

but you can find a good used bike in the paper or asking around at your local shop - i bought 2 of my bikes from guys who work at bike shops -- one was my current road bike which was the bike shop owner's training bike in great shape as he's a bike mechanic too - he bought a new bike and transferred the new components onto the old frame (he had super-nice Campi) and sold it to me and spend $900 instead of $1700 for my aluminum race-ready road-racer (OK snobs, it's only a Raleigh R700 with Sachs/Campi new Success but it works for me).

for your budget i would DEFINITELY recommend a used bike b/c you will go so much more for your money. there are always guys upgrading who want to get rid of their 'old' bike that is still in great shape. When i was poor in college i bought my 1st 2 bikes used and got great bikes at great prices - $300 for my first road-racing bike (Giant road-racer - this was 1990 pre-MTB mainstream) and then in '92 my first MTB for $350 ('89 Stumpjumper w/ all XT - it was 3 years old for an original proce of about $1200) ... with inflation, $500 should buy you about the same thing now i think.

as for which bike... i agree with almost all of the comments and it's a personal decision depending on what you want to do - and even harder b/C you don't know if you want to ride trails or tour or road-race or what...

i personally would recommend a MTB b/c it's the most versatile. you can use it for touring, commuting, road-riding and of course off-road. basically the only real disadvantage would be if you start doing tons of miles on the road and training and road-racing in which case you'll probably want/need a better road-racing bike than what you'll get for $500 (OK, not necessarily as i raced on my $300 used Raleigh with old stryrofoam helmut and got laughed at at all the races b/c i didn't have any fancy 'roadie' gear until i passed most people on my squeaking piece of crap - i was poor and couldn't afford any upgrades and just buying lube for the chain was breaking my budget so my bike was always squeaking and in crappy condition as i did all the maintainence w/o tools or knowledge or replacing broken part - i made them work)

if you buy a road bike, it will be a less-ideal commuter(potholes, curbs, rack-mounts), not good for touring -- tires too thin, gearing too high (yes it can be changed), and no rack-mounts, etc... and you obviously cannot ride offroad as knobbies won't fit on a road bike and the brakes suck

a cross-bike it an option... but harder to find. a good mountain bike for cheap should be easier to find.

buy a good MTB hardtail now with a decent frame, quality components (Shimano Doere or LX mix or better), quality suspension fork (just no super-heavy or walmart-special that looks cool but doesn't do anything), with vbrakes... after you know more what you want to do you can maybe buy a road bike or cross-bike or whatever...

for almost everything except racing you can use the MTB and you'll just be a little slower... the gearing thing isn't much of a factor unless you're racing and you can buy a set of high-pressure slicks for pretty cheap to make the MTB pretty road-worthy.

P.S. i still have my '89 Stumpjumper i bought in '92 although my first road bike got stolen... and i still have my last MTB ('97 Norco Torent) which would be a great bike for you in your price range except i'll never sell it...