I have been working as an intern for one years, You should have finished at least two semester including summer semester, to apply for CPT, it is kind of part time working permit. You can intern at any time without CPT if you are ok with no payment. In my point of view, there are two kinds of interns; Intern with payment, or without payment.
As far as intern with payment is concerned, you should get a CPT(Curricular Practical Training). The requirements for CPT are as followings:

1. You should have finished at least two semester.(summer semester can also be regarded as one semester)
2. Employer Letter
3. Profesor Letter
4. Internship Course registration.