You are referring to an externship, not an internship. An internship is a year of oral surgery spent after dental school. This is typically done by people who did not match and are trying to bolster their CV for the following cycle and/or by those just trying to gain more exposure to the field to see if it right for them.
Generally speaking, programs do not charge you to partake in an externship. However, you will have to provide your own transportation there and often, your own lodging and meals. Although, residents at many programs are good about hooking you up with meals off of their meal cards. Of course, you don't get paid.
Most often, students go on externships during their clinical years. Summer between D3 and D4 and spring break of D3 are great times. You can go on externships earlier at a select few programs, but it may only be observational.
To get an externship, you first want to decide where you want to go. In that decision, you may want to figure out if you want a program with significant training in malignant path or not. If you aren't sure, that is what the externship is for and you will want to go to a program that does quite a bit of cancer, e.g. Michigan. I would suggest heading to a program or two that you think you want to attend, but I would do this after having been on an externship or two so that you are not so green. You may also want to extern at a program known to give externs good, hands on experience as well, e.g. LSU-S, LSU-NO, or Knoxville to name a few.
Once you know where you want to go, I would first look on the program websites to see if they tell you how to apply. If not, just email the program coordinator or director and express your interest. They will walk you through the application/scheduling procedure. The earlier you book, the better because the good months may fill fast.
Good luck
