I've taught beginning adult guitar with another instructor a few times. It's nice if you keep the class size down to 6-10 students so you can assist individually. If the group is small it's nice to ask what ideal goal each person would like to achieve. That can really focus the direction of the instruction.
For a beginning class you should at least lightly cover what makes an instrument play easily and explain how that will help their progress. A first lesson should also cover how to tune. I throw a couple of strings out to demonstrate the importance of a tuned instrument. Rank beginners may or may not know these basics and you can learn how to tailor your material by asking a few questions about prior experience and any efforts up to the present.
I like to cover the concept of 1, 4, 5 chord progressions and the fact that somewhere in the vicinity of 75% of popular songs can be played by learning only 3 chords, and learning the relative minor for those three chords will boost that to about 90%. A beginner's class should be based upon learning those 4 simple chords and a few songs should be charted out to demonstrate (and learn). Lessons can start with a 2 chord song to prevent anyone from being discouraged.
I like to teach the concept of cut and waltz time and back that up by teaching bass / chord and bass-chord-chord strums using a single chord, usually a G, as I try and teach the 1, 4, 5, relative minor idea using G, C, D, and Em.
Those concepts can be verbally expanded by explaining that those same principals apply to other keys, with a demonstration of a 4 chord song in 3 or 4 other keys to show what can be done in further classes.
If you can provide printed materials that's a big plus, and charted material that uses songs that is appropriate to the group is nice. There are a lot of resources out there on the web, so it's possible you won't have to re-invent the wheel to teach your class.