Put together an age-appropriate setlist for the gig. Select songs to appeal for the group you are playing for. For example, for children select songs such as "Old MacDonald" or "Row, Row, Row Your Boat." A more middle-aged audience may appreciate old rock songs such as "The Night They Drove Old Dixie Down" or "Friend of the Devil." If the setting is a more traditional folk-style venue, select traditional folk songs such as "This Land is Your Land" and "If I Had a Hammer."
Write out the lyrics in large letters with the chord changes over them. Make a booklet or folder for the songs, and arrange them in alphabetical order. Find the right key to sing in that suits your singing voice. It is difficult to lead a group in sing-along if you are unable to sing the songs yourself. An easy solution is to use a capo on the guitar. For example, "The Night They Drove Old Dixie Down" is in the key of C. The chords are Am, C, F, and Dm. All the chords can be played in the first position of the guitar, the first three frets. Place a capo on the second fret. You can still play the same chord shapes, but the key is D major and the chords are Bm, D, G and Em.
Practice playing and singing the songs at home. Sing the songs in the order that you want to present them. Practicing the songs at home builds your self-confidence and prepares you to lead sing-along sessions with authority.
Pay attention to your audience as you play. Give the audience vocal cues to help them remember the lyrics. Divide the room into sections or groups, and make each section responsible for a particular verse or chorus.