Usually the first session with a mediator is at no charge and is kind of an evaluation on the mediator's part to see whether mediation is likely to be successful for you. For example, if the two of you can't sit in the same room together and have a civil conversation, they'll probably tell you they can't help you.
Otherwise they'll probably lay out their process. It will probably involve each of you laying out what you want, so you should be prepared to articulate that (probably not in the intake session but in a subsequent session once you've got them on retainer). But it's not necessary, I don't think, to have any specific discussions with your husband about what you're seeking before you lay it out for the mediator.
If you live in a community property state, you're each entitle to half of the marital estate. If you each have IRAs or other retirement accounts, unless they're equal one of you will probably have to transfer some funds to the other. If either of you has a pension, either drawing currently or in the future, that will also be divided according to how much of the pension was earned during the marriage.
If the mediator is good and experienced, they should be able to give each of you an unbiased view of what's fair and what's within the law in your state, and help you come to a mutual agreement.
Even if you're already mostly agreed on the division of property, etc., you'll probably still need some help getting everything written up in the proper form and filed with the court. Usually a mediator can help you with that as well--if the mediator is not an attorney they'll usually have a paralegal on staff who can do the legal paperwork.