Just a thought, but if you don't know what you want and your funds are limited try to borrow a bike from a friend. You may even try to find a good used one at a thrift store. Look for aluminum wheels and substantial componets. The cheap stuff tends to rust and bend. If you only pay $5-10 for a bike and it's not right, just keep the rims and get something else. Learn to work on it yourself. Brake pads, cables, chains and tires need to be changed regularly. Don't try to upgrade, start saving for a really nice bike. Then when you walk in to buy a good bike you can talk intelegently with the LBS. They make bikes in different sizes, so get used to measuring to see if the bike in question is the right bike for you.
Good luck