When you set out to design your own house, it is best to start with a small house plan. You can always add to it once you get the basics down on paper.
You have the choice of choosing a house plan from someone else, or design your own house plans. If you are doing it for the first time, a good idea is to find several house plans you like and then sit down and make your own which combines the best of all of them.
You want to design your own house plan such that you have room to grow. Will you be having more children? Might your parents be coming to live with you someday? These are things you need to think about way in advance. Also things like how many stories you want your house to have. You can easily climb stairs now but how about in 30 to 40 years? Do you want to have a bedroom (or maybe even the master bedroom) on the ground floor?