I'm a bike addict! I am also licensed by the League of American Bicyclists to teach bicycle safety classes.
I hope you can get a bike. Buying a used bike from Craigslist can be tricky if you don't know what to look for. It also can give you a great bargain. So, if you have a local college stop by the sports department and ask if someone can help you when you shop on Craigslist. Either a student or professor who has some knowledge of cycling.
Stay away for the big box stores. They are great for many things but not for buying bikes. The bikes there have not been put together by bike mechanics so there's good chance they are not put together properly.
I would recommend you look into a hybrid. Mountain bikes can be very heavy because of the hydraulics. They also have those nubby tiers that are made to ride on dirt. A hybrid will have tires made to ride on pavement. Who wants to be riding around on a bike that has features that you don't need?
Get on YouTube to get more information about cycling. There are loads of videos that will teach you had to fit your bike or how to change a flat tire.
Go for it! I LOVE riding my bike.