Design
The first place to start with any kind of building project is in planning. You should have an idea of what your goal is when you're building your LEGO house in order to make the building to your specifications. It may help to make a rough sketch of the building on paper, or perhaps use LEGO's Creator online. See the Resources for the Creator link, and for further suggestions and ideas for your house. You can also make many other types of buildings with LEGO; your are only limited by your imagination.
Foundation
Always start with a base tile. These provide a large, flat surface to start building on. The first layer is one of the most important, as it dictates the size and shape of the rest of the building. If you want to play with minifigs and items inside the house, you are going to need to build some kind of access for your hands, so take that into consideration. Some builders make the house on two half-plates and connect them with a large hinge, so they can open the house up and play inside.
Structure
Build the house using your first row of bricks as a guide. Be sure to offset the bricks on the next set of rows, to reinforce the structure--if all of the pieces are lined up on each other, your house will be easier to break. If you are building the house with a large hinge, make sure you're building a clean seam between the two half-houses, or else the house won't open correctly--you should open the hinge a little bit to make the gap more apparent.
While you're building, add the doors and windows where you want them to be. The average height of a LEGO person is a little under four bricks high, so your walls should be at least that tall. If you are adding any furniture or people at this point, do so when you have the first floor assembled.
Extra Floors
When adding a second or third floor, use as few flat plates as possible. If you have a particularly large house or room, you may want to make a second, offset row of plates for extra stability. For rooms with large open spaces below, or especially if you're adding a wall above an empty room below, add support columns.
The Roof
Make the roof out of sloping bricks. You can add columns for extra support, or a spine in the center of the roof: when you bring the sides of the roof together, the very top of the roof can be covered with sloping plates. The space underneath that top-spine can be partially filled with narrow plates.