Hi there,
first you need to hold a PPL yourself (minimum age 17).
Flight Instructor - JAR FCL (Flight Crew Licensing):
Instructor Ratings
An instructor rating is valid for 3 years. To be allowed to begin a Flight Instructor (FI(A)) course the pilot must have 200 hr of flight time of which 100 hr must be PIC, if the pilot is the holder of an ATPL(A) or CPL(A), 150 hr PIC if the holder of a PPL(A) and have passed the CPL(A) knowledge test.
Hope I could help.