My first year was pretty course heavy, and as PhDToBe said, not very different from undergrad in that regard. The biggest difference in the classes for me was that all my classes were small, I had them with the same 6-8 people, and it actually matters what the profs think of you since you'll have to work with them for the rest of your program. You can't slack, but there's no reason to be a super overachiever or do unnecessary work, either, since classes aren't the most important part of grad school. It's sort of a balancing act between doing enough but not too much.
Also, if you start thinking of grad school more as a job than school, you'll be better off. You can't skip classes or meetings, and you can't just goof off when you're not in class. Your adviser may expect you to be around the lab for a certain number of hours each week. Basically, you can't be flaky anymore. Although first year may sort of feel like it, this isn't undergrad. The great thing about being a first year is that older students will hopefully be very willing to help you out, give you advice, and generally welcome you to the program (if your program has this kind of environment--mine does). Good luck and have fun!