I would recommend that the first thing you should do is go into schools and get some voluntary experience.
I'm a teacher, but I know TA jobs aren't easy to come by, especially with all the budget cuts, schools are cutting back. Where once every class had a TA, now one TA is shared over several classes. Having a good chunk of volunatary experience will give you a good start, and it will also get you 'known' as it were. In my experience, if a school does need to appoint a TA then it is usually someone they already know.
Colleges do various courses - there are NVQs (I think!!) in being a Teaching Assistant, it would be best to look at the prospectuses for your local colleges. HLTA courses are something different - I don't know much about them.
Good luck