You will be asked to referee papers, look at graduate applications, look a faculty applications, write reports and recommendation letters. You will be asked to sit on committees: curriculum committees, space committees, program committee, web page committees, budget committees, planning committees and other committees you would never have imagined. You'll have committee meetings at the departmental, divisional and university levels as well as committees to serve the broader theory community. You will be asked to organize workshops, conferences and edit special issues. You'll also be teaching, writing grants and going to faculty meetings.
All of the above are good things to do. But do all of the above and you'll never get tenure. You need time for research. So learn to limit yourself, learn to say "no". I'm not saying not to do any of the above. Most of these tasks have to get done; you should do your fair share and be a "good citizen". But you don't need to agree to every request, feel free to turn down requests when you feel yourself getting overloaded. If you gain a reputation as someone who can't say "no" people will take advantage of you. And don't fall for the "You are the only one who can do a good job" ploy.
If you try to do too much you won't do anything particularly well. I would much rather have someone say "no" to me than saying "yes" and doing a mediocre job.