Most TVs have an optical audio output. If your surround sound system has an optical input then you need a toslink optical cable. On many TVs you have to turn it on and the TV speakers off to get it to work.
If you have a headphone out on your TV you can use a cable with a 3.5mm stereo plug at the TV end and 2 RCA plugs at the surround sound end. Won't work as well.
There are other scenarios where you would need an optical to coax digital converter or a DAC to convert optical to analog. TV audio settings and surround sound mode would have to be set correctly.