if i want to change a sprocket bike to a smaller what would i have to do?

[weezy5040]

ok so if i want to change my bike sprocket to a 26 what would i have to do to the bike explain it easy please like show me one thats a 26 in a eastern bike and i know u would have to buy somthing for the back so what would i get for the back what is it called just show me everything i need on danscomp please