"When do we eat? I am hungry!"
Translation:Wann essen wir? Ich habe Hunger!
Wann: when, as in "Wann ist mein Geburtstag?" (When is my birthday?), wann is used for making questions relating to time.
Wenn: can also be translated as when, but is more used in the sense of 'at the time when' or 'if' (notice that there are also other ways to say "If" in German) e.g. "Wenn du dein Zimmer nicht aufräumst, kriegst du kein Eis!" (If you do not tidy up your room, you will get no ice cream!). "Wenn ich dir etwas sage, musst du das machen." (When I tell you to do something, you have to do it)
In German you don't say 'Ich bin hungrig' (I am hungry), you say 'Ich habe Hunger' (I have hunger). Same with thirstiness. 'Ich habe Durst' rather than 'Ich bin durstig'.
I'm not too certain as to the exact reason why, sadly. It's just what I've picked up whilst learning at school years ago without it being explained.