You finally got a new job, now what?

My question is, what are some of the most important thing you can do in your new position? You can't assume that the way things are done in your new company will be the same as they were at your previous job.

How does one blend in with office culture when joining a new company? Any tricks or pointers?

And what should one focus on? e.g. department culture; upper management style; personality etc.