Hi - welcome to PP and congratulations on your pregnancy.
I suppose each doctor is different, but the ones I know of, where you don't know the date of your last period which is what is used to calulate the due date, they will wait before you get the forms and stuff until you have had a dating scan. These are not always accurate, but generally speaking an embryo grows in a predicatable manner up to a certain point, cells dividing etc, and taking average measurements can work out a DD for you.
If you do know your dates then all you have to do is count 40 weeks, or use a chart, you will find one on the web somewhere or in any pregnancy book. The dating scan may then differ from what you believe but they generally only change your date if there is more than a five day difference between the two. As only 5% of babies are born on the DD it is just a guide and for admin purposes.