Choosing a major is quite a challenge for some people, especially those with a wide range of interests. At the UW, you generally need to declare a major by the time you have earned 105 credits.
The Undergraduate Advising Office, part of the Center for Undergraduate Advising, Diversity & Student Success (141 Mary Gates Hall) has a workshop offered frequently throughout the year on "How to Choose a Major and Career." This hour-long group session helps students to zero in on their interests and strengths, and is a good starting point for those in the process of selecting a major.
The link to the Center is: http://depts.washington.edu/mgh141