Hi there,
You should try having a look around previous threads, or use the search function, this one's quite recent.
http://www.pprune.org/flying-instruc...nstructor.html
Also read the relevant sections of the CAA publication LASORS.
http://www.caa.co.uk/docs/33/srg_lts...%202010%20.pdf
Specifically, section C (PPL) and/or D (CPL) for the licence you will need, section F about the rating for the type or class of aircraft you wish to fly, typically SEP(L) and section H for instructor ratings.
Also have a look at some of the bigger flying school websites, as I'd think they will have options and explanations in among the sales pitch.
But basically, to give you a start, I'm guessing you have no flying experience or qualifications at the moment;
You need some kind of pilot's licence, private or commercial, both involve flying and theoretical training of different levels and amounts.
A medical, class 1 for a CPL or class 2 for a PPL.
Your chosen licence (which entitles you to fly something) will come initially with a class or type rating (which specifies exactly which aircraft you can fly), usually for single engine piston landplanes, which are your typical flying school Pipers and Cessnas.
You then, subject to the correct experience, do an instructors course and get an FI rating.
The current situation requires an FI to hold a professional licence to be paid. The regulations are changing in April, after which the new rules will allow you to be paid with a PPL, so you need to be looking at the post EASA requirements.
After you've done a bit more research yourself, come back with any further questions you have.