Since you've already been accepted to the school. email the coach now! Tell him your interested in walking on, and let him get back to you.
DONT, and i cant stress this enough, tell him how hard you throw or what pitches you have in the first email. Feel free to give him your height and weight, but don't say what your pitches are unless he asks. He wants to see that your more interested in being on the team, than how good your stuff is.
For tryouts they are going to want to see that your in great physical shape, don't be surprised if they do a timed two mile, and expect you to get it in under 12 minutes. Most likely the coach will have a mini tryout in the fall, and then preliminary cuts. whoever is left will play fall ball. Then spring time the real tryouts will begin. Be prepared for very physically demanding tryouts. They could include lifting, running, and lots of drills.
They also want your arm in shape, be prepared to throw against hitters.
What every coach does is a little different, this is just my experience.
Also many coaches has their own prerogative. I know for a fact that my head coach, at a DIII school, recruits his team, and is close minded to letting people who he hasn't really recruited on the team, even if they are better than some people that he recruited.
Walking on to a DI team is very hard, and be willing to switch positions if they ask you too. Just give it your all, and good luck.