Obtain a correctly-sized bracket either online or at an electronic retail store. Any major electronic retailer should be able to help you with this purchase if you have questions. Generally, brackets come in a size range. This means you can buy a bracket that fits a range of TVs.
For example, you might purchase a bracket that fits 32 to 56 inch (81.3 to 142.2 cm) televisions. Any flatscreen TV within this size range should fit the bracket, unless otherwise stated.