Well I would start with either mollies, platties or guppies. Bettas take a little more energy and a lot more tanks to keep them and raise them to adult.
The first thing to really consider is where can you get rid of the offspring. LFS may take them but not all will. Chain stores rarely will take them.
The next thing is don't use those plastic breeder traps. They stress out the fish too much. If you don't want to get a lot of plants, use the net breeders with some Anacharis
/elodea in them for hiding spots for the fry.
Feeding is easy, just crush up fish flakes to a fine powder and the fry will eat it readily.
Now the important thing, tank size. I don't know what size your plastic tank is but two 10 gallon tanks are really needed to breed with any true success. One for the adults and one for the fry. A 20 gallon is best so you have room for a breeder net and the adults and also so that you can have more than 10 fish in the tank.
10 gallon 20 gallon
Guppies: 25 60
Platties: 10 25
Mollies: 10 25
Convicts: couldn't tell ya but they do require a little more work to breed and to keep the fry alive
All the numbers are a combination of adults and fry, not adults. Overcrowding will lead to bad water conditions and very few if any fry surviving.