These are very big questions!
As to what to teach them I advise you to buy a series of books such as Reward, Cutting Edge (not for teaching individuals), English File, etc. You can find a longer list in publisher's catalogues: OUP; CUP; Macmillan/Pearson Education; etc. These are available online. When you get to the catalogues check out the EFL sections. You should choose a series aimed at the age group you are intending to teach (children, teenagers, adults). Make sure you also buy the accompanying cassettes and Teacher's books.
As to how - I can only assume you have little knowledge of language teaching so I suggest you find a CELTA course or equivalent. If one is not available near you or if it is too expensive you could buy and read a book such as 'How to teach' by Jeremy Harmer. However, if it is at all possible, I strongly advise taking a course.
- For Teachers