There are five closely-related keys to any major or minor key. Closely-related keys differ from a key by one flat or sharp at the most. So, to find the closely-related keys to a given key signature:

1. Determine the key
2. Identify its relative major/minor
3. Add a sharp / remove a flat, and identify those major / minor keys
4. Remove a sharp / add a flat, and identify those major / minor keys

For example, given the key of C Major:

1. The relative minor would be A Minor
2. Add a flat: F Major & D Minor
3. Add a sharp: G Major & E Minor