After you have declared your major, you will spend approximately two years working with your major advisor to fulfill your major requirements and any remaining curricular and degree requirements. Within your major you can undertake an Honors thesis, and you'll have more opportunities to participate in faculty research. Talk to your major advisor and others in your department or program for more ideas to round out your experience.
Many students study abroad or do an internship after they have declared their major. Remember, no more than 48 credit hours in a single subject field may be applied toward the 120 credit hours required for graduation, so you still have chances to experiment with electives and explore new areas of knowledge.
If your next steps after graduation involve graduate or professional school, discuss these plans with your major advisor or a pre-professional advisor. You may need to tweak your academic plan to make your applications more attractive to these programs.