A good idea is to plug something else into a nearby socket in the same room - eg: turned on lamp - and then (unless you know which fuse/circuit breaker isolates that circuit) turn off your electricity at the main switchboard. It's a good way of making sure the power is off, BUT if you aren't sure, get someone who is knowledgable about these things to assist you!
There should be 3 wires. Red = live, Black = neutral and either a bare copper wire or it may have a green and yellow sleeve on it, which is earth. Have a look at the back of the socket. It will probably have stamped in the plastic which connection is which. If not, looking at the back of the socket, the top hole is earth, the bottom left is live and bottom right is neutral. Cupra is right though; make sure you pull the fuse or switch off all the electrics.
Is the socket is a normal 13Amp twin it is likely to be fed with 2 three wire cables.
Red wires into the terminal marked L
Black wires into the terminal marked N
Earth (Yellow Green) wires into terminal marked E or one with earth symbol on it.
If in doubt turn ALL electricity off to all the house not just the socket you are working on. You may have to buy or borrow a torch though.
Good luck!
Don't forget that in newer installations Red (live) Black (Neutral) and Green (Earth) have been replaced with Brown (Live) Blue (Neutral) and Green/Yellow stripes (Earth).
Kelvin, the blue, brown and green/yellow system was introduced many years ago for plugs but not for domestic (ring mains) systems which are still red, black and green (or bare).