How you would improvise an accompaniment in the key of A minor using the information provided?

It completely depends on what information is provided. If you have a melody, you can harmonise most lines using quite basic four part harmony. If so, you could also use inversions, most commonly being 1st inversion chords.

If you have chords for comping, then just comp to the complexity that you can manage. I suppose it depends on what kind of music it is. If it is jazz, you could add 7ths, 9ths etc. and even throw in some tritone substitution.

Ultimately, more information is needed to accurately help you. Regardless, for improvising accompaniments you should have a knowledge of harmony, be it jazz or standard classical harmony.