Before the session, when you are not under the pressure of sitting there and being expected to produce for your T, decide one thing you would most like to talk about and write it on a piece of paper. Bring the paper to session with you and if you get tongue-tied, just read off of the paper, "today I would like to talk about X." And then go from there. The first sentence is the hardest.

At my sessions, I choose what we speak about and begin. This is because my T wants to be sure we talk about what I need to talk about, not what he guesses I need to talk about. I do not think it is unusual for a T to want the client to do this. If you aren't used to it, it can be stressful, though!