How do u get apps for ur unlocked/jailbroken iphone?

i bought an iphone that isalready jailbroken and unlocked, with the "installer" application on it. the thing is this "installer" doest have more than like 5 applications for me to instal. so does anyone know how to get applications on the iphone other than using the "installer" application. also, i had a theme on my iphone and i set it (its some simppsons theme) and now when i want to change it, the wallpaper or anything wont change. no matter how u do it. it is either a simpsons wallpaper or nothing. does anyone know how to change that?