+1 on TartanFrog's comment.
I've been diving for 10+ years. All wetsuit diving. About 75 dives in cold Monterey Bay water (approx 50-55F at surface) just went through a training class this summer, purchased my suit in August and I've been out now a few times.
I highly recommend training or experienced mentorship. I feel like I learned a lot from the PADI DVD. I purchased this online as my LDS would have had to special order and it would have taken too long. And I got a lot of tips while diving from the instructor.
I don't think you can have too much training or advice when it comes to diving. The worst case is you've "wasted" a day in the water. Shame.
My instructor was also willing to do a condensed course. I watched the DVD and filled out the quizzes in advance on my own. And we did the pool dive (actually in open water), and then two check-out dives, all in the same long day. Throughout the day we were able to talk diving, and drysuits and I was able to build up some knowledge there.
I didn't wind up buying the suit with this instructor because of distance from my home. And also because I thought he didn't have enough experience sizing for the suit I was interested in buying.
I know you aren't asking about equipment here (that will come next). But trilam and neoprene aren't your only choices now. I wound up getting the new BARE SB System. I'm really happy with the purchase and the recommendations and sizing I wound up getting from my not quite local LDS. The outer material on the BARE SB is a new material that is stretchy and breathable, but has the thickness (I should say thinness) of trilam.
I've been diving in the suit a few times now and really love it. I'm amazed at the end of a warm day (on the surface) with lots of swimming, that the undergarments are completely dry and there is a dampness to the interior of the outer layer.
Don't work for BARE. But couldn't be happier with my purchases.
Here's a video of the suit in action from this recent weekend:
Eric Elia