If I were you, I'd apply to DA offices, U.S. Attorney's Offices, and federal judges. The DA office / USAO will be familiar and give you exposure to what you are leaning towards at the moment. An internship with a judge will expose you to other areas of the law.
If you get the opportunity, you could split your summer.
So... "general first steps"
1) Identify the city/cities you would want to work in.
2) Google for contact information for DA, USAO, and judges in those cities.
3) Send cover letters and resumes to those offices. Identify when you will be available for interviews. Do it now. No need to wait for grades.
4) Follow up with a phone call in 1-2 weeks.
Working for a judge will be best to "keep your options open," so as to not make a law firm next fall think you are "too" interested in criminal law (big firms typically do not hire law students to join criminal defense practices). This should not really be a big factor in your 1L summer decision, though.
I think it's best to apply before you get grades. Law school grades can be kind of random. Er, at least, I never really knew how I did in a course until I received a grade. Other people disagree on this, though.
Edit: Good luck!