Have fun!! I love yoga. I agree about looking for a class for beginners, and get there a little early to let the teacher know this is your first time. Vinyasa classes move through the poses pretty quickly, which can be harder to follow for a beginner. Don't be afraid to modify the poses if you can't do the full expression, I still do that years later. And don't be afraid to take child's pose if you get exhausted.
Some studios will have workshops just for beginners, too, so you can go through the poses slowly and get alignment tips. There are always tons of Living Social & Groupon yoga deals around here (i'm in the Boston area too) so you'll probably be able to try out a few studios and styles before finding one you really click with.
It's about commitment, not motivation.