Seeing as we just went through this with another "newbie", please read this:
Newbie Help I'm clueless, Violamaster Prima?
All the advice given there applies here as well. Assuming you know nothing about basses, you will have to:
1. find someone who will teach you how to play one ("teacher").
2. obtain expert advice about buying a bass from the aforementioned "teacher" and/or from a person who is in the profession of selling basses ("luthier").
3. figure out what you want to accomplish by playing bass. Do you want to play classical? jazz? bluegrass? some combination thereof? Only you can answer this and the answer will help dictate what kind of bass you need.
Finally, "cheap" and "double bass" are mutually exlcusive terms. Expect to pay a minimum of $1.5k for a decent beginner bass.