A musician needs to memorize all the related major-minor pairs as this is very useful for songwriting and improvisation. Feel free to create a mnemonic to make it easier to remember. I memorized these just by playing them over and over, worked well for me 🙂

C Major  – A minor
D Major – B minor
E Major – C# minor
F Major – D minor
G Major – E minor
A Major – F# minor
B Major – G# minor