Can fish from supermarket be used for sushi?

This probably sounds really stupid and all, but forgive me for not knowing too much about sushi, except that it tastes good and is raw. I wanted to know if sushi is specially cleaned and prepaired before being served, or can you buy fish such as salmon from the supermarket, clean it and use that to make your own sushi?

it really depends on where you live..for the most part, a fish market is best, not only becasue its fresh, but they can help you get the right cut...some markets can get super fresh fish, but its worth it to talk to someone in the fish dept