You will, first and foremost, need to be qualified and experienced in the field in which you wish to teach. After that, you'll need to get yourself a PGCE in adult/post-compulsory education, for which you need a degree or equivalent as a prerequisite.
It's a long time since I did my PGCE, but when I did it, I was able to do it 'on the job' (you need to have a placement for the course in any case).
You dont necessarily need a PGCE - there are City and Guilds options too, but yes you will need to be qualified and experienced in the field you wish to teahc and then need an adult education of some sort
Presuming you already have a qualification / considerable experience in the field - speak to your local college about this. City and Guild entry routes are becoming more popular and you can usually do them while in service at a college.
Perhaps try the C&G 7303 PTTLS (preparing to teach in the lifelong learning sector) course first of all - it's a short course to give you a taster of teaching and may help you to springboard into a work placement / teaching job at the same college you do your PTTLS.