Learn three chords on a guitar. The 'open' chords G, C and A are the most popular for beginner guitar players because the changes are fairly easy. Learn to strum rhythms - this is the difficult bit because you're doing two things at once and it's hard to judge if you're really producing a regular rhythm or you're being a bit random. Practice strumming and changing the chords smoothly. Then when you have that mastered, just play a sequence you like to a rhythm you like over and over again, and the words will come to you because the mind 'hooks on' to something repetitive and starts to create around it. The other way around is to write a poem and then write the chords around it. Either way is good.