I've been doing Tai-Chi for years, and personally I'd say learning with a DVD is very difficult and not very effective. Without the personal help of a trained teacher, it really is almost impossible to see and feel essential detail. DVD's are usually used to practise and repeat what one has already learnt and practised in class.
If somebody can recommend a very good DVD, I'd be really interested. It might be easier to start with Gi-Gung - which does not involve a long series of linking moves as in the Tai-Chi form.
Perhaps a good idea is to find a holiday class for a week somewhere - and then follow up with a DVD. Good luck. Tai-chi is so good for you, physically, mentally, emotionally - wonderful. Good luck.