Determine the guitar riff that you want to learn. Listen to acoustic guitar songs that you enjoy and choose one that you'd like to learn. When finding your first song, try to find a song that has an easy chord progression. Listen to the song and determine how many chord changes it has and the speed in which the song is played. If there aren't that many chords or the song seems simple to play, you should choose that song as your first song to learn.
I have heard how giving you are in so many respects of music schooling and I must say that I am impressed. You remind me of the pure idealism that we had in starting Apple. If I were young, with time, I'd likely offer to join and help you in your endeavours. Keep making people happy, not just in their own learning, but in the example you set for them.
Electric tuners are easy to use and very accurate. Hold it to the guitar and pluck the high E. The tuner will tell you if the guitar is "sharp" (too high) or "flat" (too low). Pick each note and tighten the string to make it go higher, or give it some slack to lower it. Make sure the room is quiet when using a tuner because the microphone on the tuner can pick up other sounds.
Strum with loose, relaxed motion. Strumming consists of downstrokes and upstrokes in various combinations, striking all the notes of the chord evenly and rhythmically. Use your wrist to practice smooth up and down motions. Keep your elbow in tight towards the guitar and sweep the pick down all the strings. Your elbow should not move very much, as you strum mostly from the wrist.
Strum your guitar with a pick or your fingers. Hold down the strings with your fingers in the appropriate shape and try to strum with your other hand. Acoustic guitar strings often have higher actions than electric guitars, so you may have to press down very hard to get a good sound. If the chord comes out muted, try holding down the strings with more force. If your string is buzzing, move your finger further away from the metal fret on your neck.
String$: When you're done playing, wipe your strings with a cotton cloth or old T-shirt that's been treated with a small amount of light household oil such as WD-40. This cleans the strings and displaces moisture that can shorten string life. This simple process will extend string life 3 to 4 times compared to untreated strings, saving you money, and giving you more time to spend playing your guitar instead of changing strings.