I have two questions:

- Let $G$ be a finite group. Because complex representations of $G$ are completely reducible to know all representations is same as knowing irreducible ones. In case of modules over group algebra ${\overline{\mathbb F_p}}[G]$ when $p|o(G),$ we know there are possibly indecomposable modules of arbitrary large degree. In such cases when do we say that we know "all" representations of $G$ with appropriate sense the word "all"?
- Is there any way or reference to compute order of group GL$_2(\mathbb Z/p^n\mathbb Z)$? I am trying to work with modules over $\overline{\mathbb F_p}[\mbox{GL}_2(\mathbb Z/p^n\mathbb Z)].$ Is it something very trivial or very difficult?

I request all to give answer in elementary language if possible.