Why not whatever you can lay your hands on?
Ok I mean seriously, if you want to buy them.. I can't say much. I guess you can't go wrong getting DVDs from known people like Bolleteri or James I forget his name. I'll try to find some amazon reviews and such for you
Welcome to the forums!! and remember, as far as instruction is concerned, a good hitting partner who is advanced than you can do much more than instructional videos and also just getting out there and watching good people will also help you.