A good quality pellet as a staple. One which lists a protein source as the first ingredient. Omega One and New Life Spectrum (NLS) are popular around here.
Betta like a variety of food, so include some "frozen" bloodworms every few days for protein and frozen Daphnia occasionally for roughage and laxative. Don't overfeed (that's something you get the hang of with experience).
Salt should only be used as a treatment for a known condition. For bloat and other digestive problems, Epsom salt is the preferred treatment. Kosher or aquarium salt is used for skin conditions. Some highly-experienced members (Byron, for instance) do not recommend using salt at all.
Good advice from Fred on plants. If you only have Wisteria and swords. You don't have to get too fancy with ferts and light. Anubias, Java moss and Java fern are also easy to care for.
While you're waiting to decide what to do, you might consider cycling your tank. There are several good stickies near the top of the Tanks and Habitats
Betta are weird that way. They can live miserably in a cold, dirty bowl for years, yet sicken and die with the best of care. It's frustrating. But while we cherish them and care for them responsibly, they live fine "happy" lives.