You'll frequently see these posts and hear advisors say "go see the SSO." SSO is short for Student Services Officer. Every department has a staff person who works most closely with the undergraduate majors in that department. When we say "ask the SSO," that's who we mean. They are the people primarily responsible for processing your major declaration, organizing events for majors, sending emails about opportunities, tracking major requirements, and so forth.
You will typically find their names on department websites under Contact Us, Staff, or Administration. Departments use a variety of titles. The most common is probably Student Services Specialist, but you'll also see Undergraduate Administrator, Undergraduate Coordinator, Undergraduate Program Advisor, and similar labels. Engineering has listed the Engineering Student Services Specialists on a single page.
It is a very good idea to get to know the SSO in your major department, or a department you may be interested in. Some questions you might ask the SSO: is there a department events or pre-major interest email list I can join? How do I declare? Does the department have peer advisors? How do I know which faculty are available as advisors?
The SSO is also your best starting point whenever you have questions about department regulations, major requirements, and so forth. If they can't answer your question, they will know where to send you.
The SSO is not to be confused with the SSC, or Student Services Center. The SSC is where you go if you have questions for the Registrar (such as enrollment problems), Student Financial Services, or Financial Aid.