The most important thing to look for is a bike that you are comfortable on. If you are not comfortable, you will not want to ride.

I always tell people to go to every bike shop in town, ask a bunch of questions, and test ride a couple of bikes. Tell them what you told us, that you want to haven't ridden for a while and don't know much about current bikes. I would probably not mention that you intend to buy a used bike. My personal policy is to never buy a bike on my first visit to any store, in fact I try to leave my check book and credit cards home. This protects from being swayed by particularly charismatic salespeople without carefully considering new info first.

Buying from a shop is really not a bad thing tho, even if ou are trying to keep within a budget. You get professional help getting a bike that fits you, you usually get a free tune-up or two, some even offer free flat repairs, to make your first season pretty much trouble free, and you get to build a relationship with a shop full of bike geeks. There are lots of choices for entry level stuff for a few hundred dollars. If you want to really connect with your bike, look for a local shop that offers basic maintenance classes so you will know how to lube your chain, pump up your tires properly, and fix a flat. This is also a good venue to find other new bike folks for riding buddies to keep you interested.

Don't even look at the bikes in the department stores. If you have the urge to buy from Wally World or X-Mart, just search these forums for department store bikes and read the opinions. This is especially true if you don't have tools or the ability to work on bikes yourself.

You will probably want to start out with a hybrid bike or a no-frills mountain bike with an upright seating position. As you ride more and learn more about the wide world of bikes, you will develop a better idea of what kind of riding you like and want to do more of, and decide what your second and third bikes will be.