You've gotten lots of great advice above. Here's a great site to help with any fish questions: http://www.aquahobby.com/
I do want to warn you to be very careful about getting fish from a pet store. Pet stores don't usually take good care of their fish and have regular Ick outbreaks and other issues. If you don't have a good fish store to get your fish from then make sure you always quarantine any fish you bring home from pet stores before adding them to your tank. (Of course, it's always best to quarantine any fish from anywhere before adding it to your tank.)
Also, someone mentioned that bettas are often raised in quart jars (or small tanks); yes they can be but IMO it's cruel. It would be like throwing you in a small bathroom for your life. You could live there but it wouldn't be healthy or mentally stimulating.
I do want to warn you to be very careful about getting fish from a pet store. Pet stores don't usually take good care of their fish and have regular Ick outbreaks and other issues. If you don't have a good fish store to get your fish from then make sure you always quarantine any fish you bring home from pet stores before adding them to your tank. (Of course, it's always best to quarantine any fish from anywhere before adding it to your tank.)
Also, someone mentioned that bettas are often raised in quart jars (or small tanks); yes they can be but IMO it's cruel. It would be like throwing you in a small bathroom for your life. You could live there but it wouldn't be healthy or mentally stimulating.