Typically any image editor has a simple 'one stop' feature. which is the best to use because in you 'lighten' an image the the contrast/gamma has to be adusted in equal measure, this can be a messy business if you are inexperience- as I judge you are by the question. For example PaintShopPro has drop down menu -Adjust Photo. the first obtion is 'One Step Photo Fix' This applied all th options in qual measure automatically, andbe uses repeatedly until the appriate level isachieve . You then use'Save As' and givinng the image a new name to save that version of adjustment, thus saving the original in its original orm for alternative work if the first try was not to your liking.
Get Paint Shop Pro 7.0 (which is an earlier version) for free from click here. Download & install the patch as well. (PSP7's own Help should be enough, but if you need more, there are several tutorials click here)
My eyes (I find) are the best judge for a pic's colour/brightness/etc. Calibrate your monitor for the best results click here (more info click here). G
P.S. I also use another free program, Paint.NET click here, Picasa (also posted by hzhzhz) & 2 other free ones, but I would advise to keep it simple for the time being. Picasa would be a good addition & shouldn't complicate matters (I don't think!)
Most imaging programs will allow you to adjust contrst, brightness, and if necessary apply sharpening. Adobe Photoshop Elements2, as supplied with many scanners, etc, produces results like this. This took only a couple of minutes - taking more care will produce even better results.