Under JAR FCL / JAR OPS there's various categories of airline instructor / examiner, vis:
Groundschool instructor
Synthetic flight instructor (SFI)
Crew resourse management instructor (CRMI)
Line training captain (LTC)
Type rating instuctor (TRI)
Type rating examiner (TRE)
Revalidating type rating examiner (RETRE)
It is possible to become technical groundschool instructor, SFI or groundschool CRMI whilst still a first officer, but you should expect to have several years experience.
For the others you would need to be an experienced captain, then work up through LTC, TRI to TRE.
The minimum requirements for SFI, TRI and TRE can be found in LASORS. LTC is an airline appointment, but you would need to do the line training element of the CRMI course.