1. One's thoughts, feelings, actions in different situations can sometimes be generalized to understand ones thoughts/feelings/actions in the situation that you've gone to therapy about. So you do not need to know the direct relevance to think that the therapist might be able to find a connection. Sometimes I will bring something up because I think there is a connection, but sometimes when I've brought up issues for some other reason there will be a connection. So I don't have to worry too much that anything I bring up will be irrelevant. Of course, not everything I bring up will lead in lock step to a solution to my presenting issue, but therapy can't be a linear step by step process anyway.
2. The things one learns to take for granted growing up are relevant to the way one experiences any issue now. So any current interaction with foo is relevant to discuss to help t understand how you grew up (of course, the way you interacted will have changed over time, but a lot of it stays the same). Memories and feelings about family of origin are also relevant.
3. The way you interact with the therapist can sometimes be generalized to how you interact with others, so your feelings about interacting with the therapist are relevant to discuss. If you tell the t your feelings, it will help her understand why you act the way you act. then you can figure out how that relates to the reasons you went to therapy.
4 If there are immediate crises going on, obviously tell t about that if you want her help dealing with them. Even if you dont want much immediate help, telling her about immediate problematic issues helps her understand how you react to problematic issues, which she might be able to generalize to understand how to help with the reason you came to therapy.