First of all, for all bike related questions, go to bikeforums.net for good advice.
...as yesno said, it depends completely on what you're intentions are. Beyond that, it's a bike, after all, ...try not to over think it. Overspending vs underspending, as with cars, is completely up to you. That having been said, don't go the dumpster diving route; if you were to buy the most amazing bike in the world, it'd still be cheaper than almost any car. Go only to a local BIKE STORE, not a target or some sporting goods store etc. The local bike store will sell you what you need, hi or low end. You might get away with spending $100 or less from a friendly store on a used bike, but expect to pay $500 or more for a new one that's any good.
I started out with a Specialized Crossroads (so you can look it up...it's the same as any hybrid, really) bike. It's a fine bike, but it's basically a mountain bike with a cushy seat and larger wheels (the main selling point for hybrids, bigger wheels! with smooth tires)...It was fine for about a year. But in the midwest, the wind is insane (I've heard that too about Chicago....) and you begin to want more of an aerodynamic experience, rather than complete frustration when you're little more than a bitchy kite...so I went to my local bike shop and they set me up with a bike designed for cyclocross, which is great for city commutes. It's like a REAL hybrid between a road bike and a commuting bike. A strong road bike with fenders and lights, and a rack, etc. I was at first totally against drop handlebars, but quickly came to love my current set up. I thought drop handlebars were just for racing, but they're much more comfortable than upright hybrids or cruisers for long distance commutes. I've got a front wheel generator hub which powers my lights, and is completely unnoticeable as I'm moving along (nothing like bart simpson's generator, in other words...), I've come to both learn and love "clip in peddles" which allow you to use the full 360 degree motion of the peddles and stop your feet from slipping when it rains. They help you quickly gain momentum when the light turns green as well, or when you're making a left hand turn...pulling up with your legs helps for sure.
I suggest starting simple at a local bike store such as one Dania suggested. They'll more than likely have a road frame with fenders etc for sale. I advise you to avoid "hybrids" and Amsterdam style cruisers, especially if you live in any city in the USA. Even Portland I'd avoid them as a commuting bike. They're too heavy and aren't that comfortable for commuting. When I was getting into biking I was wishing I could find bikes like they have in Amsterdam, I thought they'd be more comfortable etc, but in the USA we just don't have the infrastructure for such bikes. Better get a light, strong, road bike with fenders etc such as the Bianchi Castro Valley, or an old steel framed schwinn from the 80's off ebay with some fenders and flashing lights on it...
Don't forget your helmet and plenty of lights, FLASHING LIGHTS, both for yourself and for cars seeing you.. If you see people without a helmet, that's their problem. Wear a fucking helmet.
here's a picture of my bike. I've ridden 4500 miles now on it since April of 06. No problems. Surly Cross Check frame. ...notice the reflective tape, reflective tires, and the best show sticker for luck...
Whatever bike you end up getting, be safe and good luck!
