Google Code-In 2018

Want to know ,how could one apply to be a mentor for HAIKU in GCI 2018

Usually we do not accept people directly as GCI mentors. You need to already be involved in the project in other places. Being a mentor means you represent the project to contestants, and this is something we can put only in the hands of trusted people.

You are however welcome to join the IRC channel or help around here in the forums, where the students will most likely be looking for help.