If you're really unsure of yourself its best to do a bit of research before you go diving in. Mirek's suggestion is a good one - there are indeed lots of books out there by some pretty amazing artists. A couple I remember being particularly good at showing the basics right through to a decent end product are:

How to Draw Comics the Marvel Way: by Stan Lee
Figure Drawing for all its Worth: by Andrew Loomis

Some pretty darn famous artists giving away their top tips, its good stuff.

Another way could be to try sketching people from magazines or newspapers, and see how you get on. Whatever you're more comfortable with really. But ultimately it's pretty hard to get semi/full naked people to pose for you at most times, so as soon as you're comfortable I'd get to the classes one way or another.