Adding a light and outlet to new room
 

I'm adding a walk in closet in what is now unfinished attic space. It will be attached to a bedroom in the finished side of the attic and it's above a living room. I need to "tap in" to existing wiring to add the switch and outlet, preferably the wiring from the bedroom so that it's on the same breaker. I have an idea of how this is accomplished, but I was hoping someone with experience could explain it so that I can be 100% confident with what I'm doing.

Take your new 12/2 or 14/2 wire from a existing receptacle box in the bedroom, run it to the switch box for closet, then your 14/2 from switch up to the ceiling light box. Hope you understand. ;)

Thanks again GregC. It makes perfect sense, I'm just hoping that I can find an exposed receptacle. I imagine it's the same process for the outlet?

Welcom...You got it. Just loop together, as long as you don't have too many drawing from the same then you get into all the volts vs amp thing and have to possibly up the size of the breaker.

After doing a little more reading, I have the wiring from the source to the light, switch, and the outlet figured out. What I'm not 100% sure of is how to know if a certain receptacle is a good source of power. If I have an outlet with live/neutral/ground connected, can I simply run new 12/2 from the unused connections on that receptacle? Alternatively (maybe better), there is an outlet in the BR near where the attic is that I will never use. Can I remove that outlet and use that wire as a source?

You could get a clamp-on ammeter
http://news.thomasnet.com/images/large/008/8194.jpg
and measure the typical current through the breakers in your panel that are handled by your candidate outlets.
Pick an outlet served by a breaker that typically runs well below it's rated 15A or 20A capacity.

If an outlet can run a hair dryer at full speed and full power, the wiring connection integrity is good all the way back to the panel.

Sure, that would work. You got it! Either, Or. Pull off of that receptacle or remove it entirely and use it as your source. ;)

If you are just adding a switch and outlet to a closet, I'd use 14 gauge wire vs 12 unless the circuit has a 20 amp circuit breaker. The heavier wire is much harder to work with.

If you take off a resi. panel cover you should be wearing level 2 arc flash gear. Getting a shock may be the lesser hazard.

Are you serious? This gear would cost more than hiring an electrician to perform the work on a one-time installation.