Step1: First of all you need to know the size of your TV.
Start from the top left corner of the screen (ignoring speakers and the frame) and measure diagonally to the bottom right of the screen.
Step2: Know you know the size, you need the VESA. The VESA is the distance between the holes on the back of the TV.
VESA is the distance in millimetres between hole 1 and hole 2, then measure the distance between hole 2 and hole 3.