You need to go to each major’s web site at UCLA to find out what the procedure is to declare the major. Usually, it means taking the prerequisite courses for the major. If the major is more popular than its department can handle, there may be a high college GPA requirement and/or competitive admission to get into the major.
For example, http://catalog.registrar.ucla.edu/ucla-catalog20-21-1230.html lists the requirements to declare the psychology major.