For future reference:
This is a VERY easy upgrade.
If you want to DIY, there are two sites to know for Macs. The first is Otherworld Computing (macsales.com). They generally have good prices on memory... really, on most Mac stuff. You can find better prices, but always make this your first stop. Why? Because even when it has the same specs, not all memory is equal in the Mac world. OWC makes sure that their memory works in the Macs, and they make it easy to get the right memory for the Mac you have, including telling you if there are any special considerations (such as needing matched pairs for the memory).
The second site: ifixit.com. They have step-by-step instructions for replacing parts on... well, on almost any kind of tech you could want to know how to fix. You look up your exact model, and you should be able to find illustrated instructions on putting in the memory (although with that model, it should be pretty simple.)
The Apple Genius bar is another good resource - but buying the memory there will likely be a lot more expensive than you could get from OWC. I'm not sure if they will help install it if you bring it in from elsewhere.
BTW, according to OWC, you should be able to do up to 16GB. I would recommend putting in the max amount if you can.
One last thing - if you are having speed issues, make sure you have some free disk space. The system needs at least some free disk space for swap, and the closer you are to using all the storage, the poorer your performance will be. Often, a good disk cleaning (or upgrading to a larger HD) can do as much for performance as adding memory can.