Make out a list (Quicken is great if you already have it) of EVERY bank account, credit card, mutual fund, cars/boats, house(s), stocks/bonds, property, loans/mortgage, you name it. You need to assign a value to these things and if they are jointly or individually owned.
Make a photocopy of everyone's birth certificate, social security cards, and driver's license.
Inventory the contents of your safety deposit box.
Meet with several top-notch lawyers in your area. In my area lawyers do not have free consultations, in other areas they apparently do. The reason to meet with the best is so that your husband doesn't have one of them representing HIM.
You will very likely get life-time spousal support (formerly known as alimony) due to the long-term nature of your marriage and that you were a SAHM. But this will depend on your age and ability to find a job to support yourself, so don't be surprised if the possibility of NO spousal support is at least discussed.
You'll want to make sure that your children's college education is covered in the paperwork. Who's going to pay for it? Also, in some states children who are in college still get child support. Find out about that.
Find out about you getting to keep the house, at least during the divorce process. This is called "exclusive use". For a SAHM (this is me, too!), knowing you can stay in your home while you have no way to pay for an apartment or even someone's couch to flop on is a big deal.