The relative minor of a major key
lies three semitones below: C - Am; D - Bm; A - F#m, etc.
As the name suggests, the two keys are closely related: for one thing, they have the same key signature. Modulation into the relative minor is far from difficult, and might merely involve changing a cadential chord in the original key to one in the new key. eg, C - Am - F - G into C becoming C - Am - F - E7 into Am.