If you are asked to write a minor sixth above the note $\text{A}$, start with the $\text{A}$ major scale and key signature. In $\text{A}$ major, there is an $\text{F}^♯$, which is a major sixth above the note $\text{A}$. Therefore, $\text{F}^♮$ is a minor sixth above $\text{A}$.
Alternatively, you can simply count the number of half steps. If you know there are 8 half steps in a minor sixth, you can count from $\text{A}$ up to $\text{F}$.