There have been a number of threads on this so if you do a search you'll find more than I can tell you.
To instruct you need an FI rating. To get the FI rating you need to take a 30 hour course. Before you start the course you need a minimum number of hours and to have demonstrated CPL-level knowledge, usually interpreted as having passed the 9 CPL or 14 ATPL exams.
To get paid for instruction you'll need a commercial licence (CPL) and a class 1 medical. Again the exam requirements as above, and once again a minimum number of hours.
I do it part-time but still manage about 200 hours a year. I could never live off PPL instruction full-time!
Good luck