Select a floor plan that will fit your LOT, WISHES and BUDGET.
Think of everything your house has to include, and write it down.
This list should include the number- and size of rooms and other spaces:

Entry Way / Foyer
Living Room
Dining Room
Kitchen
Breakfast Room
Family (Great) Room
Den / Library
Music Room
Powder Room
Laundry Room
Storage Room
Master Bedroom
Master Bathroom
Master Closet
Guest Room
Home Office
Play / Exercise / Sewing
Bathroom
Other: (Hallway Width), Attic, Basement, Garage, Carport, Work Area, Storage, Front Porch, Walk Way, Drive Way, Deck, Patio, Screened Porch

Then find a floor plan here http://www.theplancollection.com/house-plan-styles/ that comes close to being ideal for you and use your imagination to make it ideal. Move some walls, make one room bigger, make another smaller, add a C.R., play with the doors, windows etc.

Of course an engineer or architect can give you a lot of money-saving tips like the fact that a wall on the groundfoor that goes straight up until the second floor and roof is cheaper than a wall on the second floor that is not resting on a wall under it; it needs extra bracing under it.

Verify also the orientation of your house- and window positioning/alignment with the sun, shadow plus wind (this effects the floorplan).
Then layout the floor plans on site (on your lot with some wires and lines) so that you have the physical feel of the room sizes and the house as a whole. It is better to revise the room sizes etc. on paper, rather than wait for it to be constructed…