I was diagnosed when I was just twelve and its already been ten years, dang. Let me tell you what I learned about sharing with friends about MS.
First of all, you are going to know right away who is your friend and who isn't and that is probably the best thing about having MS, you learn so much about people!
When I was first diagnosed, I didn't tell anybody, even within my family, it was rarely spoken of. My attacks were pretty bad the first couple years and I didn't want to talk about it ever. I had a hard time moving away from that, during high school, it was like my dirty little secret, I think I told three people. It was horrible, I don't recommend that, I was very sad and bitter most of the time. Its funny how, at that age, it is so hard to be different because right once high school is over, being different makes you who you are- someone people are interested in knowing.
Once I got to college, it was a clean slate and I decided to embrace the fact that I have MS. My friends and roommates were incredibly accepting and it made a world of difference. I started to develop into the person I am and I'm sad I didn't realize how to do so earlier.
I'm still a private person and I don't just tell everyone in general conversation but if it comes up, I'm not afraid to speak up. The way I look at it is, its not a secret and its better than having people talk about it behind your back. It took me ten years to learn that though, I'm still careful who I tell and how I tell them without giving away too much. Over time, you'll learn what works best for you. How I usually bring it up when someone asks or I feel like it is time to share with them is like this, "I'm not sure if you are familiar with it but I have MS."
Anyway, good luck, hope it helps and sorry it is so long!