Be sure it's something you want to do. The market is already saturated with terrible instructors who only got into it as a last resort when their CPL/IR didn't immediately land them a job.
It's a very tough job to do well, and is certainly something you need to be passionate about.
Providing you are passionate about flying light aircraft, you will love instructing. The going rate is about £7k all in. There is a LOT of groundschool on the course which usually accounts for a significant amount of the fee.
Do some research into instructor pay. You're lucky to get 500 hours a year in some schools, so try to make some contacts now, or ask outright if you'd get a job on course completion when you do your FI rating.
I cannot emphasise enough that you need to be sure it's what you want, otherwise you're just giving a really bad service to some poor students, or you simply end up doing trial lessons all day every day and never converting them into a full course.