Have you settled on a design style you like?
 

Take a drive around the area you are considering building in. Make note of the styles that appeal to you. If you don't know a colonial from a contemporary, you can still take a photo or two, and perhaps some notes about elements you like, such as wraparound porches or dormers, bay windows or cupolas. Bear in mind that the most attractive houses have a consistent style and relate in some way to the surrounding homes.
 
Do you have some favorite materials?
Cedar-shake roofs appeal to some, while inexpensive fiberglass shingles are fine with others. Maybe you have your heart set on brick, but have you considered what color brick and which texture you like? Once again, the best way to refine your choices is to look for houses you like, and note all of the materials they use and the way the materials are combined.

What configuration appeals to you?
This is really a two-part decision. You may love the thought of a split-level or a ranch, or a raised ranch home. On your drives, you'll be looking for appealing examples of your favorites.
But have you considered whether your proposed location will work with that type of plan? Many one-story plans require a wide lot. The typical split-level house settles best onto a lot that slopes from side to side. If you are thinking about a walkout or daylight basement, you should be looking for a lot that slopes down in back.

Finally, bear in mind that challenging sites -- the boulder-strewn outcrop or postage-stamp size city lot -- may mandate a house with a small "footprint."