Most single coil pickups only have two wires. A negative / start lead and a positive / finish lead.
The negative lead needs to connect to the common ground circuit of the guitar, which is usually the back of a potentiometer.
The positive lead connects to either your pickup selector switch or the input lug of your volume pot, dependant on how your control circuit is wired on your guitar.
Sometimes the pickup will have two physical wires going out of the pickup. The black lead is the negative and the other wire, which may be various colours to denote the pickups position, will be coloured.
If you think there is only one wire coming out of the pickup this will actually be a twin cored wired. The outer section of the wire is the negative and the inner core of the wire is the positive.