Watch the Building I, IV, V Progressions online guitar lesson by Jason Loughlin from Focus On: Travis Picking
I'll give you three approaches to easily locate the I, IV and V chord in a key. The single string approached lets us find the root of the I, IV and V chord all on one string. You can do this off of any string. Find the Root of the key and move up two whole steps and a half step. That's 5 frets higher. We call that distance a fourth. This is our IV chord. Our V chord is always a whole step higher. That's 2 frets higher. The 6th string approach means we are starting with our root on the 6th string and trying to stay in position for the IV and V chords. So instead of jumping around on one string I'm going to look for the IV chord on the 5th string. It's super simple to find. The IV chord is directly underneath on the root on the 5th string. The V chord is always a whole step higher. The 5th string approach has us starting with our I chord's root on the 5th string. Now we're going to look for the closest IV chord. You'll find this by moving your I chord root down a whole step(2 frets) and then moving straight up to the 6th string. The V chord is a whole step higher(2 frets).