Seeing the posts of bravery of putting cold water and tropical fish together... Don't do it. Especially if you are new to keeping fish in general!
Depending on the betta, you can have certain fish such as guppies, platys, some tetras, cory cats, etc. depending on the size of tank, and the temperament of your betta, the results will definitely vary. Certain fish (such as the cory cats) should have more "floor space" than say guppies. But some bettas get very offended at the sight of a guppy (it's like a small, colourful male betta to them) while others do not care. Some bettas will also get very stressed, may get sick, or may be bullied.
Avoid fish that are very nippy such as rasbora or barbs. Many people I know have African Dwarf Frogs (not to be confused with the African Clawed) with their betta fish without problems. Allow ample hiding spots, plants, etc, and they get along fine.
Breed for the breed, not for the money; the words any REAL breeder would understand.
You must know nothing in order to know all, for all is nothing and nothing is for all.