I am assuming you mean a new light to be installed outside, and getting the feed from an existing light switch which operates an inside light.
The first thing I would do is turn off the power, remove the cover and loosen the switch so you can see what is in the switchbox. I would say that in 80% of switch installations there is no neutral wire in a switchbox. If that is the case, it is usually easier to get the feed from another location. Also, check your local code to see if a GFI circuit is required for the outside light.
If there is a neutral and GFI is not an issue, you need to install a larger box for the 2 switches and run a two conductor with ground of the correct size (based on the breaker for the circuit) to the new light location. Your new white will splice to the existing neutral (don't assume all white wires in the box are neutral-could be a switch leg). You can take the main hot feed and splice two jumpers to it, one to feed each switch.
If no neutral, you need to tap into a different source and install a separate switch at a new location. If GFI is an issue, you may need to tap into the breaker box if there is a GFI breaker you can use or possibly tap in downstream of an exiting GFI, or install anew GFI to connect to.
The first thing I would do is turn off the power, remove the cover and loosen the switch so you can see what is in the switchbox. I would say that in 80% of switch installations there is no neutral wire in a switchbox. If that is the case, it is usually easier to get the feed from another location. Also, check your local code to see if a GFI circuit is required for the outside light.
If there is a neutral and GFI is not an issue, you need to install a larger box for the 2 switches and run a two conductor with ground of the correct size (based on the breaker for the circuit) to the new light location. Your new white will splice to the existing neutral (don't assume all white wires in the box are neutral-could be a switch leg). You can take the main hot feed and splice two jumpers to it, one to feed each switch.
If no neutral, you need to tap into a different source and install a separate switch at a new location. If GFI is an issue, you may need to tap into the breaker box if there is a GFI breaker you can use or possibly tap in downstream of an exiting GFI, or install anew GFI to connect to.