When selecting a position on the wall where you can mount your TV, you must consider the following four points: the proximity to electrical outlet, the proximity to TV signal line, if the wall is solid and strong for mounting the TV and not in direct light.
Use a stud finder to locate the timber studs behind the plasterboard, and fix the brackets there for strength. You can mark the positions with a pencil or stick some paper in the spots as shown. If the wall is solid, use the extra-large wall plugs that come with most bracket kits.