the major scale is a set of intervals - half and whole steps. it's easy to see on a piano keyboard -- if you don't have one, draw one (a couple octaves). the major scale goes from C to C on the white notes only. as in, C-D-E-F-G-A-B-C or whole-whole-half-whole-whole-whole-half. this is also the ionian mode.
now, the other modes can be learned the same way. dorian is also whole and half steps, just in a different order. on the piano, it's from D to D on the white notes (which would be W-H-W-W-W-H-W). phrygian is from E to E. lydian is from F, mixolydian from G, aeolian (also known as minor) from A, and locrian from B.
the guitar is obviously not arranged the same way, but if you can locate these notes, you can learn these scales. once you've done that, with your keyboard (or drawing) write out the patterns of whole and half steps (or you can find several online) --- then try starting any given mode on a different pitch. for example, locrian is the SET of whole and half steps as it would be starting on B on the piano and only playing white notes, but you could start it on... G#. any pitch.
this website has the guitar chords for each mode (links are in the left column):
http://gosk.com/scales/major-scale-for-guitar.php