Can you declare a major in your first year?

Yes, you can. If you know what you want to major in, you can and should start taking classes in that field as soon as you get to college. But you don't need to declare a major to take classes in economics; some people are considering several majors their first few years, so when they declare they have hopefully already completed some of the requirements for the major (since most can't be done in only 2 years). No, it's not going to help you graduate faster. Although picking a major with a lot of requirements and not starting them until junior year won't help you graduate on time.