Most grammar books designed for ESL that I've found usually start with the verb "to be". Especially if they are true beginners. It would be impossible to start teaching people the ideas of a sentence/kinds of sentences etc. if they have a very limited knowledge of English. The important thing here is first to find out their levels. Are they beginner/intermediate/advanced etc.? Once you have assessed their needs you can begin selecting material and working with it. If you are serious about teaching English, you should really consider some sort of certification. I've learned that is isn't just enough to speak the language to be able to teach it. There are lots of theories out there and you can eventually choose which theory or philosophy of teaching you agree with the most.