Locate the right MIDI sound. You'll want to find a sound that mimics the percussive plucking and decay of a guitar string. This is a percussive technique when performed on the guitar, so it is important that you choose a sound that relates to it. Some MIDI modules have guitar sounds in them. These will work just fine.
Familiarize yourself with the technique. A guitar hammer-on is accomplished by fretting a note with one finger on the left hand, striking the note with a pick in the right hand and then "hammering" an adjacent note on the same string with another finger on the left hand.
The most important characteristics of the sound that this creates are the percussive attack of the first note and the much softer attack of the second note. Guitar players will often apply a bit of vibrato to the second note---this can be accomplished with a MIDI controller either with a pitch wheel or by applying an effect in a sound-editing software suite (see Resources).
Apply what you know about the guitar technique to the MIDI controller. Choose the notes you want to play. Try a basic passage from a major pentatonic scale, as this scale is very common in guitar music. Strike the first note hard and then hit the second note more softly while letting up on the first note at the same time. It will take some time to master the technique, but this should get you started.