Like you said start slowly. Try to form a good relationship with your T; this will make things better and easier.
You can beat around the bush and allude to the csa, or you could say right out. If you take the later path, how much you want to disclose is up to you. You could start off strong and say, "I was sexually abused and it still hurts."
Where you go from there depends on you, the T and other factors, but no matter what you have the right to say 'no'; 'No, I don't want to go into detail right now', etc. On the other hand, you may like opening up, but then again, it depends.