It has been raised before if you use the search above.
To be paid you need a CPL, an FI rating and the CPL theory exams. You can work without pay on a PPL with CPL theory & an FI rating.
The terrible situation in the airline industry has prompted many to become instructors while waiting for the airlines to recruit again, meaning there are now few instructor jobs in Europe and North America. Unless you are thinking long term, secure the job before you start the FI course.