Openness:
The more open you are able to be, the more you will benefit from the sessions. For some people getting into deep feelings can be difficult and scary and that's OK. That's probably been a very useful survival mechanism which has served you well in the past. However, often people come to therapy because their old ways of being are no longer helpful and avoiding emotions can be unhelpful, if you want to have a rewarding relationship with a partner or with friends. So being as open as you feel able to be will help you get used to connecting and sharing, which is a great foundation for all the other relationships in your everyday life.
Working with conflict:
As a therapist that does not mean that I will always get things right, sometimes I might not understand properly, If you are unhappy with anything that has occurred in your session, I encourage you to talk about it. This can really help you become more assertive in your life. To be able to disagree with someone who will work with what you bring non-defensively can be transformational and break old patterns of pleasing others or avoiding conflict.
Honesty:
Try to be as honest with your therapist as you can. Often people come to therapy having spent a lifetime holding back how they really think and feel. Use your therapy sessions to really bring what you are thinking and feeling - it could change your life!