There are two main types of instructor – Flight Instructors and Class Rating Instructors. Which course you decide upon will depend largely on what you wish to do with your rating once achieved.
In order to give instruction to ab-initio students for the grant of a PPL(A) you will need to be a Flight Instructor. This rating is the usual course undertaken by the majority of people wishing to instruct and tends to be the rating that most flying schools will require you to hold before employing you as an instructor. You can instruct on a PPL(A) but you cannot be paid unless you hold a commercial licence.