This is a good question. I still have a really hard time with this.
As a lot of people advise, it's good to write down what happened before you plan to tell someone. If you can't physically tell them, you can also hand them the paper with your writing.

If you're not sure what exactly happened to you, tell them that. Maybe you could do some research beforehand, or talk to someone on the rainn.org hotline and they can help you figure it out. I went there for help when I wasn't sure if something I experienced qualified as csa and they helped me figure out the words and process it a little, so I could talk to my T about it later.

What helps me when I talk about it in therapy is to hold something in my hand like a rock or silly putty. The energy from my discomfort and stress goes into whatever I'm squeezing, so it gets a little easier for the words to come out.