I believe regulations recently changed and it is now compulsory to have a teaching qualification before teaching adults, although I think this can be done while you are teaching if you are qualified in your subject to at least Level 3 (A level). Most colleges would want you to have done/be doing a basic teaching qualification - I think there are 3 stages of City and Guilds you progress through - starting with 20 hours teaching at stage 1, then 40, then 60 at stage 3 which is equivalent to PGCE. This is at least two years training part time. Best contacting your local college as they will know best.
If you're really keen it's worth volunteering to gain a bit of experience before starting to study and check you really do want to do it! Good Luck.