Apply for internship online, include resume and transcript. My process was slightly different than the average person's, and I was never formally interviewed. The only call I got (months later) was to ask if I was still interested in the position.

They said that they hadn't gotten my resume yet from HR, so they wanted me to verbally go over my resume. Talked about their organization and it's role in the FAA, work flow and culture. Then we had a few basic technical questions. Some of them were kind of random, but nothing especially tricky. They asked me to evaluate myself in several areas (technical, social, time management, etc.). We discussed personalities and made a little pop-culture small talk (since me and the two interviewers were roughly the same age). It was very relaxed and friendly.