Installing LED Strip Lights
You need a power supply which provides 12 volts DC. Given a sensible margin for safety and expansion, I would go with a 


15 watt unit for 1 light
30 watt unit for 2 lights
45 watt unit for 3 lights.  


The main thing is making sure the power supply unit is legal to connect in bousehold wiring, depending on how power is being supplied to it.  It might be a unit that bolts into a 1/2" knockout, or mounts on a junction box faceplate, etc.