There are over 1200 clubs at Cal (academic, arts, cultural, music, political, professional, religious, service, and sports). Although your studies are your top reason for being here, having an extracurricular outlet helps you stay healthy, maintain sanity, and make friends. Even if you get on lots of groups' listservs, select only one or two to actively join. It's not a good idea to over-commit and end up compromising your academics.
Cal Corps, Stiles Hall, the YWCA, and other recruitment or retention centers offer dozens of ways to give back to the local and national communities, such as relief work in New Orleans, tutoring middle school children, or outreach to high school students about going to college.
Service work connects you to a community, and can help keep you grounded in where you come from.
If you need a part-time job, look for one with flexible hours so you can fit it around classes. Check the listings on the Career Center and Work-Study websites. Look into the libraries, Recreational Sports department, residence halls, and the
Student Learning Center, which tend to have lots of jobs. If possible, limit yourself to 8-12 hours of work per week to ensure you have enough time for your studies.
After a couple semesters, try stretching yourself to take on a position of increased responsibility.
You might become a coordinator for your student organization, a supervisor at your job, or get involved in student government. You'll make a valuable contribution to the campus while also strengthening your professional skills for when you graduate. You can always start by taking a leadership class like CALeads.
Don't do too much. One week has 168 hours, and you need 50-60 to sleep, 40-50 for classes and studying, and 20-30 for eating and personal errands. Spend the remaining 30-40 hours wisely; maybe a part-time job and one club, but leave time to spend with friends, having fun, and not living every minute on a schedule!