Tu : is a pronoun (je,tu,il/elle/on,nous,vous,ils) and is only used before a verb. ex: Tu manges des tomates (you are eating tomatoes)
toi: when you want to say "you" and there is no verb before or after.
Toi, tu pars à 10h (You, you are leaving at 10am)
I hope hope this makes sense to you (i am bilingual so this should be right lol)