Every major key has a relative minor. Every minor key has a relative major. Finding the relative major or minor is actually pretty easy.
Want to find the relative minor? Go down a minor 3rd distance. A minor 3rd distance is a whole step + half step. In other words, subtract 3 from the fret you are on, and that's how far you need to go.
For example: to find the relative minor to the key of C, start on a C note. You will start on C because C is the root note of the key of C. Let's use the 8th fret on your low E string for our C note. Now, move down 3 frets. You will land on fret 5 which is an A note. That is root note of the minor key - the key of A minor.
Want to find the relative major? Go up a minor 3rd distance. In other words, add 3.
For example: to find the relative major to the key of B minor, start on a B note. Let's use the 2nd fret of the A string. Now, move up 3 frets. You will land on fret 5 which is a D note. The key of D is the relative major to the key of B minor.
Check out the following video for more examples and details on how to find the relative major and minor:
John Taylor - guitar and bass instructor for Mile High Shred.