For Video, you will need one cable known as "Composite".
It plugs into a Yellow RCA socket on your TV, which looks like this:
You will need a cable with one of these plugs on each end:
Then, for sound you"ll need a cable like this one:
These plug into the Red and White RCA sockets next to the Yellow one on your TV.
At the RPi end, the Yellow RCA plugs into the RCA socket and the 3.5mm stereo jack plugs into the stereo jack socket. There is one of each, so you can"t get confused.
Should cost well under a fiver for the full set of cables, you can get them from places like PC World and Maplin.
(Don't bother getting "Gold" ones, it makes zero difference in a home environment.)
If your TV doesn"t have this set of Yellow, Red and White RCA sockets, look for a SCART socket, and you can get adapters for SCART to give you the Yellow (Composite Video) and Red/White Stereo Audio RCA sockets.
These look something like like this, and cost about a pound: