If it's in the key of A Minor, then you can use the chords diatonic to the A Minor scale, as well as substitutes and leading chords (but if you're trying to stay on the safe side, stick with the diatonic for a while). For a natural minor key, the diatonic chords are as follows: i, ii(dim), III, iv, v, VI, VII. So the chord built off the first note in A Minor, which is A, will be minor, the second note (B) will make a diminished chord, the third (C) will be a major chord, the fourth (D) and fifth (E) will be minor. The sixth (F) and seventh (G) will be major. Notice that these are the same chords you can use in C Major, A Minor's relative major key. You can apply the formula (i, ii(dim), III, iv, v, VI, VII) to any minor key for easily useable chords. Hope that helps. Later.