Existing power from light to other sources
As it was pointed out your question lacks a little detail, but I will tell you this. You need to think of running a lighting circuit from switch to switch not light to light. 

First the new NEC requires you to have a neutral in every switch box.

Second if you start your circuit at the switch box you can now run it to either one light or multiple lights on one switch or even multiple switching points. If you are running directly to light without switching them then consider the fixture a switching point.

Third if you are extending from an existing light circuit, be sure and do a load calculation to make sure you don't overload a circuit.

Good Luck