Federal internships are an excellent introduction to the workings of national policy. Often organizations may not advertise an internship, but could arrange one for you. Send letters of interest to organizations of interest. Remember, these individuals were once students looking for internships too and are often eager to assist you. Research the internship and its responsibilities.
Every internship is different.
Don't end up surprised or disappointed. Learn all of their expectations, and the position's responsibilities upfront. Create or update your resume early. Share it with friends, faculty, or someone in your future profession in order to gain up-front feedback. For your job search, you will also need letters of recommendation.