It's not the there are things in JPG that you can't edit (but you can with RAW). It's just that with RAW, there is more data in the image to work with. If you have dim, underexposed images, you'll be able to brighten them up with generally better results if you start with a RAW file rather than JPG. If your shots are well exposed to begin with, then it doesn't matter so much.
Most photo editing programs will alow you to do pretty much the same things with either JPG or RAW. For example, Adobe Camera Raw, which is the RAW editing program for Lightroom and Photoshop, you can do the same thing to RAW and JPG files, with a slight difference in how you adjust white balance or temperature. With many images you may get better results with a RAW file, but the parameters you can adjust are the same.
Free with the camera is View NX2, which handles data transfer, but also allows you to do some basic editing of RAW files.
Also with the camera is Capture NX2. It does more comprehensive editing. I believe you only get a trial of the software with the camera.
If you want a free RAW editor, try Raw Therapee. I've never used it, but friend rave about it.
But if you only have a few dark images, see what you can do with View NX2. It will allow you to adjust exposure and brightness.