To start with, I didn't tell anyone. I felt I could handle it myself at that stage, but I couldn't handle emotionally supporting someone else. People are different though - many people will want the support of a partner at this stage. I used the time between tests and appointments to learn about the disease, in part so that when I did get around to telling people, I would be able to answer any questions they had. One of those questions was obviously my prognosis, so I decided I would wait until diagnosis had gone far enough to give me a good idea of my prognosis before telling anyone.
First people I told were my brother and sister-in-law, and it is quite a relief when you first tell someone - you now have someone who will listen. Then parents. Then gradually I told a few friends. I would joke about it occasionally (hormone therapy could have been invented for comedians), so people would know I'm OK to talk about it, but you don't want talk exclusively about it or you might lose some friends. Just after my radical treatment finished, I told all my friends. By that stage I was fully confident talking about it, and had even been counseling other men with it.
So, take it at your own pace, and be aware of the impact it has on the people you tell. In some cases, you will need to provide them with some emotional support if you are able to do so. As I said before, being able to laugh at it can really help put others at ease.