All students must take a seminar in their first year. Seminars are small-group learning experiences that allow you the opportunity to engage closely with a faculty member and other students. Students usually take their seminar the semester they are not taking WRITING 101, but you could take both in the same semester.
There are special seminars designed just for first-year students, but you can also take any class that has an 'S' at the end of the course number, though it's usually best to stick with courses lower than the 300 level, depending on your background in the subject. You can see the options for fall first-year seminars, numbered 89S here.