Determine the size of your TV
The Society of Motion Pictures and Television Engineers recommends a 30-degree viewing angle for watching video from the seating area. The angle starts at the point where you are sitting and expands like a triangle to the points on the left and right side of your TV or projection screen. Rather than go into a lot of geometry, this simply means that the optimal distance from your seating area to your television should be 1.9 times the width of the TV. If you are considering a 42-inch flat screen television, the precise distance from the TV to your seats should be approximately 80 inches, which is 6 feet 8 inches.
A basic 5.1 home theater audio setup requires two stereo speakers flanking the TV, a center channel speaker over the TV to deliver dialog, two rear channel speakers just behind and slightly above the seating area at a 45-degree angle to the viewers, and a subwoofer to deliver deep bass. Placement of the subwoofer doesn't matter too much, since the low-frequency sound is omnidirectional. But the front channel speakers, to get true stereo separation, should be at least 10 feet apart. The rear channels should be between 1 to 5 feet behind the viewing area, depending on their size and power capabilities.
Other furniture, decorations and accessories
For some home theater enthusiasts, the seating area can be as simple as a sofa or futon flanked by a pair of easy recliner chairs. Others may demand folding seats such as found in movie theaters. In the former situation, you can rearrange the room at will, but theater seats must be bolted down. In that case, everything else in the room must be placed around the permanent seats, which could restrict your options.
If your home theater will also be used as a gaming room, consider where you are going to store or display gaming chairs and hardware, as well as media storage for your film, music and game collections. If your room is not enclosed by drywall, media storage units in front of wood paneling can help improve acoustics, as can carpeting or wood flooring, but storage units take up floor space. You'll want to plan speaker placement and the location of your TV and electronic components before adding too many bookshelves of media storage.
Before you set up everything on cement or slab flooring, consider installing carpet, which enhances acoustics. Next time you go to the movies, run your hand along the wall in the theater. You may be surprised to discover it is covered in carpet. This is done to improve the Surround Sound quality for a more realistic audio experience. The same technique can work in your home if you cover bare floors before you begin installing equipment.