I would suggest first deciding what kind of bike you'd like. Road vs. Mountain vs. Hybrid.
Road - more aggressive positioning (lean more forward) for aerodynamic benefits. Thinner tires.
Mountain - more upright, sturdy/heavy. Wider tires for offroad
Hybrid/Cross - something in between. Usually road type bike with clearance for slightly wider tires.
In your case I would suggest a used 90s steel mountain/hybrid bike. There are millions of them around and all over craiglist. Make sure the bike fits well (ie. you can adjust the seat/handlebars sufficiently to be comfortable).
A bike such as this (mountain/hybrid) should also have lots of eyelets for attaching fenders/racks/baskets to carry stuff.
Craigslist is a good way but if you're not very familiar with bikes you should try to find a friend who can help. That way they can go with you to look at any used bikes and make sure it's decent. If that's not possible, hopefully a local bike shop can help and not try to up sell you. There are many, many used bikes in your price range so don't let a shop try to talk you into a new bike.
Another option is to ask local bike riders you run into what they like about their bikes and let them point out stuff. I don't think there's a serious bike rider who wouldn't spend hours discussing bikes.
Hope this helps.