During your 4th year of ROTC, you will list the 10 branches that you want. Special Forces is NOT available to 2LT. You must be a 1LT to APPLY for Special Forces. The same goes for Rangers. The Army TRIES to give you one of the branches in your top 4. For the last few years, it has been about 80-90% that Soldiers get one of their top 2 choices. The Army does have a program to commit to extra time in the military in exchange for receiving your top choice. If you don't get the top choice, you don't serve the additional active duty time.
Whether you receive your top choice comes down to your standing on the OML - Order of Merit List. It is the class standing in ROTC and is heavily dependent on your GPA. It also takes into account your physical fitness scores, ROTC instructor evaluations, and LDAC (summer camp) course evaluation. If you have a low GPA, you may not even get active duty. That is not guaranteed.
If you are going for Special Forces, the best path for a 2LT would be to branch Infantry. Go to Airborne School during the summer between your junior and senior year. All active duty Infantry officers WILL go to Ranger School after completing Infantry BOLC. If you haven't done Airborne, you will do Airborne after BOLC, then Ranger School. Go to your first unit and be a superstar. After your platoon leader time, when you make 1LT, you can APPLY for Special Forces or for the 75th.