draw yourself 1 or two octaves of the piano. Find the note of the key you are in i.e. C Major, to find the minor you need to go DOWN (think u need to find the minor - which is sad if u get me) 3 semitones for instance 1. C-B. 2.B- Bb 3.Bb - A
Therefore you are in A Minor
TO get to a major key from a minor, simply reverse the process and go UP (think happy) 3 semitones for instance A Minor 1.A-Bb 2. Bb-B 3.B-C
Therefore you are in C Major
Hope it helps