Sorry to hear that.
Now get rid of that food,
Get a few more plants put them in the tank get some good quality pellets take your time with these if you do not have the money straight away, keep doing water changes, then in a few weeks go out and buy the Best fish you can afford, Do not pick the fish, let the fish pick you, you will know the one because he will be begging for your attention flaring at you etc. Dont worry if he looks a bit rough with the right food and water conditions he will look his best in no time, another thing to consider is a female Betta.