How can I decorate a living room with white walls? I like olive green, dark brown, and orange for color.

Nice color combo!  It would definitely look cozy with the right touches.

Depending on what type of couch color you have (chocolate brown or olive green) add a large area rug under the base of the couch, extending under your coffee table.  For a true homey touch, get a deep pile area rug so that you have something cushy to sink your feet into when getting up and down.  A Flokati rug is so soft, has a long pile and adds a touch of luxury to your room.

With white walls, you really need to refocus the eye and use the white as a back drop.  Add some color with frames and other knick knacks.  One idea is to create a collage of black or deep brown frames, blowing up your favorite photos and putting them up in a set pattern on the wall.  You might want to focus on sunsets or orange flowers, just to get that extra orange color into your room. Prints of Prairie lilies, falling leaves, or the beautiful orange glow of the sun across water would look great.  If you blow the prints up in black and white or sepia, take a felt and lightly color in a random flower or leaf orange.  It's a very beautiful effect.

Fill up the wall space by adding items such as a large clock, wall sconces that can be plugged in, or framed artwork. Once again, focus on deep greens or orange colors, so that your eye is drawn to the color on the wall and not the white.

Be sure to add orange pillows to the couch and curtains to the windows in your room. They will liven up the space with color and create an extra level of coziness. It sounds like it will be a great room, enjoy!