First. You'll need to determine where the main seating position will be. This is TYPICALLY the diagonal of the TV times 1.62 for a 16:9 HDTV. Since your TV is 51 inches, then around 82 inches will be your sitting distance from your TV.
The center channel would indeed be at the center at 0°. Left and right speakers should be located 22° to 30° to the left and right of center. The surround speakers should be at 90° to 110° from center, which would be directly facing each ear to slightly behind each ear. All speakers should be at ear level but it is also OK to have the center above or below and pointed up/down at the main listening position. The surround speakers can also be moved UP and pointed at the the listening position. Main left/right can either be pointed straight or toed in towards the listening position depending on the TYPE ( mono-pole, di-pole, bi-pole ) speaker. These are only GENERAL guidelines and should be used as a STARTING POINT for finer speaker placement depending on your room.
Just like a drug bust gone bad... Everything should be pointed at your head.