I think it's best to leave them with as few questions as possible. When I told my boss, we discussed:
1) Timeline - When to start hormones, when to change my name, if I had any surgeries on the horizon, etc.
2) Name change - When did I want people to refer to me as Max
3) Any leave usage - Did I know or have a rough idea of any periods of time that I would need off in advance
4) Coming out - When did I want my coworkers to know, when did I want my clients to know, etc.
I would say at a minimum, have your coming out process figured out as much as possible. Ask about any non-discrimination policies or policies for trans employees that are in place. If they don't have any, offer to assist in creating them.
Overall just be helpful and understanding. You are quite possibly the first trans person they'll have helped, and it might be very overwhelming for them.