Dites-moi is "tell me" (this is a kind of order) whereas "vous me dites..." is "you tell me..." (it's only stating some said something). You will use "dites-moi" a lot more than the other. You could say for example "dites-moi, quelle heure est-il ?" (tell me, what time is it?).By the way, "dites" is polite or plural, you may want to use "dis-moi" with your friends.

Dites-moi is in the imperative, where as the other one, vous me dites, is in the present.