Jonathan,
I just started my first aikido class last night as well (if you don't count a Ki Development class a few weeks ago, which really isn't Aikido per se).
I had a cold, was more than a little clumsy, and absolutely had a great time anyway.
Wish I could go several times a week, but I'll at least wait until I get through the 4-week intro class.
As far as dojo ettiquette goes, I understand it can vary a lot. Follow the lead of more senior students and you should be fine