You can use the following ways to connect a set-top box to a TV:
1. SCART on STB to SCART on TV. This provides the best possible connection, with stereo sound and "RGB" highest quality connection;
2. SCART on STB to composite on TV. Older TVs without SCART connections usually have "composite" inputs, usually three "phono" sockets (often red, white and yellow). This offers stereo sound and good picture quality. You will need a special cable from a high-street retailer.
3. UHF ("aerial lead") box to TV. This requires that the STB you choose (all Sky, MOST Freeview) has a UHF output. You will need to "tune" the TV into the box, and you will get OK pictures and only mono sound.
If you are stuck with mono or poor quality sound on the TV, you can still use the SCART output (and the same cable as 2) to a separate amplifier to listen in stereo.
Caroline Brown: Further to StevensOnln1's excellent advise, what is the model of your TV, as well as the set-top box?
If you have a flat TV, then usually you connect it up via HDMI leads, not scart, and even relatively old TV's should have a Freeview tuner already built in. If the new box is an HD one, then you'll need to use HDMI to get the best use out of it.