Prepare to take things slowly and to trust that sensei knows what he/she's talking about. I see a lot of newbies who seem to think that they know better than the person teaching them (not saying that you're like that, but you may be grouped with other beginners who have this attitude). Everything has a place and reason, even if it takes years to understand why
Also be prepared for the etiquette. Your new dojo should have a list on their website or maybe pinned up somewhere. If all else fails, have a chat with someone before class about what's expected.
Above all else, have fun!