Start by picking a fret on the sixth string of your guitar - let's try the fifth fret (the note "A"). Play that note. This corresponds to the first note on the bottom left of the accompanying diagram. Then, slide your finger up three frets, and play that note. Then, move up two frets, and play that note. And, then move up two frets again, and play that note. Now move up three frets, and play that note. Finally, move up two frets, and play that note. This last note should be the octave of the first note you played. If you counted correctly, you should be at the 17th fret of your guitar. Once you've done this, try playing back down the fretboard, in reverse order, until you arrive back at the fifth fret. Keep doing this until you can play the scale pattern by memory.
Congratulations... you've just learned the A minor pentatonic scale. Strum an A minor chord... it should sound like it "fits" the scale you just played. Now, try playing the scale again, except this time, when you get to the 17th fret, try playing up the scale one note higher. Since the first and last notes of the pentatonic scale are the same note (an octave up), you can just begin repeating the pattern to play further up the string. So, in this case, the next note of the scale would be up three frets, or all the way up to the 20th fret. The note after that would be at the 22nd fret.
You can use this pattern to play the minor pentatonic scale anywhere on the guitar fretboard. If you started the scale pattern on the third fret of the sixth string, it would be the G minor pentatonic scale, since you started the pattern on the note G. If you started the scale on the third fret of the fifth string (the note "C"), you'd be playing the C minor pentatonic scale.