Learn the notes of the harmonica from the Internet or lessons. You need to be able to hear the notes or read them to play a straight song on the harmonica. The first note of the harmonica is the key the harmonica is in. If you buy one that's in the key of C major, then the first note is C. Frequently a paper comes with the harmonica that tells the notes on that particular harmonica.
Find a simple tune that you would like to play on the harmonica. Most of the old campfire songs and classic folk songs are simple enough to learn without lessons and lend themselves quite well to the straight harp technique.
Play 1 note of the song at a time. When you study the harmonica lesson for straight harp, you notice that there are few chords involved. Again, you play it simple and straight.