So you have a switch, a light, but no cable in between them? If, in the switch, all the insulated wires present are connected to the switch terminals then, whatever the colour, they are all live wires. If this is the case then you should be able to adapt the (many) lighting diagrams in the For Reference post to your needs. Essentially, you need to pick up the lighting circuit at some exissting point and extend it to where you need it and connect in a switch cable... which you run back to the switch.
If some of the insulated wires in the switch are connected together in a terminal block then these are the neutrals and you will be able to simply run a single cable from the switch to the new light position. This is a less frequently encountered scenario.
In both cases earth wires should be green/yellow sleeved, connected together and ideally connected to the galvanised back box and/or to the faceplate of the switch if it is a metal one.
To supply any piece of electrical equipment you need, in order: A source of potential, a live conductor with a switch somewhere along it, which terminates at the load. Then you need a neutral conductor to carry the current back to the source. This is what all the diagrams describe in some way or other. If you can't interpret them then maybe you shouldn't be messing with electricity. Seriously
Another thought: If you are talking about adding a high wattage floodlight outside then you need to consider taking a spur from a higher capacity circuit. If you're not sure about any of the above, please call in an electrician. (By the way, I just spent an hour-and-a-half sorting out a lighting problem for a DIY-er who got lost and spent all day blowing fuses - I charged him £40 and he was so relieved to be able to get the lights back on before his missus came home he gave me £60! There's no shame in seeking professional help, but a lot to lose if you chance it)
