I'm pretty confused by how to make someone understand I am trying to say tué, tu or tout.
J'ai tué mon ami. (I killed my friend.)
Tu es mon ami. (You are my friend.)
J'ai tout, mon ami. (I have everything, my friend.)
To answer your question, "u" and "ou" are pronounced differently in French. However your first two sentences are incorrect, which is probably adding to the difficulty. The first should read J'ai tué mon ami and it's not totally clear what the second is trying to say.
For a real example of somewhere this can make a difference, note the difference between Vous lavez-vous? (Do you wash yourself?) and Vous l'avez vu? (Did you see it?) or of course dessus and dessous, or cul and cou...