here is a basic theory of what you need to accomplish. First, the depth of TV's are so deep that you first need to measure how deep the shelve has to be to accomodate this depth.
I would first consider a tv with little weight and shallow. But if you already have one, then you need that TV's dimensions.
I would first locate the studs on each side of the 90 degree angle of the walls.
Then I would make this sucker out of solid oak.
I would then mitre two 2 x 3's that will meet in the corner and span over to 2 studs. i would lag screw these 2 x 3's to the studs, four studs total. Make sure you center the lags dead center in the studs of the walls.
now the shelve. I would use one complete sheet of oak for the shelve. in other words not have a pair of 45's. Cut the ends to fit or follow the walls on both sides. Screw this shelve into the 2 x 3's.
Now the hard part. You need a center brace just for insurance. Design a brace again of oak that is about one foot to 18 inches long and comes out to the end of the shelve on top. now, you need to mitre or angle each wall side of this brace to fit nicely in the corner. You will automatically have a stud or two in the corner to screw to.
Then screw the top of the support to the shelve again.
Now, you should be set. I recommend oak because of its strength and durability.