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!
This is an activity that feels different than it looks.
1) Feel awkward for months
2) Be frustrated over and over again
3) Believe that you are slowing the class down
4) Feel that you will never be able to learn this
5) Have all kinds of new pains
6) Constantly battle the desire to quit.
7) Meet a lot of nice people but a few that you really won't like.
8) Constantly be told that you are doing your techniques incorrectly
These are not the universal feelings of all but they are a compilation of the feeling of all those who did in fact quit. Prepare for these feelings and thoughts and you might have a chance to make it long term.
Thanks a lot, I appreciate being prepared.