what have you tried? help is more likely to be forthcoming, if people know exactly what steps are giving you trouble.
1) hint: if every element is its own inverse, this means x^2 = e, for all x in G. play around with (xy)^2 = xyxy.
2) what is I? what exactly is "under +4"?
3) verify the group axioms for this operation. be sure to show that a*b is never 1 (closure).