You can try finding the studs as has been mentioned, but your best bet is to fix a couple of wooden battens vertically to the plasterboard using the recommended.
Can You Put Tv Bracket Plasterboard Wall. I don't know what the total weight will be but the bracket is rated for 30kg (66lbs), so let's assume the tv is the heaviest possible for the bracket. Here you will be able to see how to fix a bracket and a 40inch flat screen tv to a plasterboard wall using 6x blue gripits.
HANGING A FLAT SCREEN TV... / Ask-a-Saint | London ... from ask-a-saint.silversaints.com
Everyone loves having their tv. Predrill the holes as shown in the manual to prevent tearing. Mounting materials such as screws, concrete anchors.
Am about to fix a invision wall bracket to connect a 55inch panasonic tele.
Make sure you have the right tv bracket for the use you want to put your tv to, the height. Is it safe to mount tv on plasterboard? Put the tv back on the wall and see if it's level. The wall can be reinforced with wood and then the tv is mounted on the wood.