Well if anything is bad, its most likely the coil. But you will have it out and apart anyways so may as well spend the extra 5 bucks and do the plug. In other words I would do them both.
Basically the COP's and plugs are located underneath the silver fuel rail. They have circular tops, and are black. They look like this...
https://www.google.com/search?hl=en..._coil_2wire%2Fcop_coil_overview_2.php;261;291
There is a plug that gives the COP power, unplug that. Next there is one small bolt that just holds it down, take that out. Can't remember the exact size, but it is not in very tight. After that, the COP just slides out. Down in the hole is the plug. This is the tricky part, you need a spark plug socket (5 bucks at any auto parts store) and some extensions. Depending which cylinder it is dictates how many extensions. You will also want atleast one universal joint (2 depending once again on the cylinder in question). Push the plug socket down into the hole and push it onto the plug, there is a little rubber piece that is supposed to grab the top of the plug. It shouldn't take much torque to loosen it up, and then it should just pull out. Try and clean out the hole as best you can before doing this as you do not want too much debris to fall into the cylinder. I use some compressed air to do this. Put the new plug in the same way.
MAKE SURE IT DOES NOT CROSS THREAD!!
These heads are aluminum and therefore the threads are easy to mess up. When I have done them, I tighten them down fairly snug, but not as hard as I can. There is a certain torque spec out there that I am sure someone will post up, but I do not know off hand what it is and haven't used it anyways. New COP goes back in (should slide right onto the plug, you will see what I mean when you do it), and then just the small bolt and plug and she'll run (almost) like new.
When you get the codes, if it is one of the front cylinders (1, 2, 7, or 8) I would do it yourself. The rear ones are a lot more tricky as you cannot really see what is going on, and need to be creative with extensions and u-joints, etc. That will be left up to how confident you feel.