In order to instruct you must have at least a PPL with 150 hours PIC (200 hours total) and have demonstrated knowledge at CPL level i.e you have passed all the CPL written examinations. Under JAR-FCL there is no division of Techs and Navs however, most people elect to take them in those associated groups.
If you take the ATPL exams as many will advise you to do, there is about 3 times as much work, and once taken, they are only valid for 36 months to obtain both a CPL and an IR (Beware!)