in a gallon tank, with a Betta, you would be dooming any fish to decapitation and a horrid death. the Betta thinks this tiny world is his, and he will defend it with his life if he has to.
while you may not be able to make a fish community, you can make a decent critter-community. a pair of Glass Shrimp, an African Dwarf Frog, and a few Ramshorn Snails will do greatly. the snails provide eggs and babies for everything to snack on, but you will have to cull the population every so often. if you do this, you'd also need to get some sinking Algae and Shrimp tablets, and (if you don't have it already) live, frozen or gelled Bloodworms.
i'd suggest saving up, going to the nearest LPS (not a chain store, a small, reliable, LPS!) and looking around. usually small stores will have simple glass tanks chock full 'o stuff-you-need. i saw one today for $30CAD; a five gallon tank, gravel, heater, filter, hood, light, food, conditioner, net, 'Everything You Need To Know' booklet, thermometer, and pHAdjust. you can often find a better one than what big companies like Hagen are offering for cheaper, and they do well. in a five gallon you could move your Betta, add in a school of Zebra Danios or Neon/Glowlight/Red-Eye/Black Neon Tetras and the Shrimp/Frog/Snails and you have a beautiful, thriving community.
