First, a quick note about digital photography: Unless you're going to be printing pictures bigger than A4, you'll never need more than 3Megapixels on any camera (because you won't be able to tell the difference). Do a bit of experimenting and find out which settings on your camera siut your photography and give files of about 0.5Mb Anything much above this is just wasting space on your media card and increasing the time it takes for your camera to store each picture.
Having got that out of the way, I'll try to answer your question!
If you've got Windows XP, then you don't need additional software. Just follow the instructions here:
http://www.steve.maurer.net/tutorials/XP_tutorials/xp_re size_pics.htm
If you've not got XP, you need some extra software but you might have something suitable already. Check the help files for whichever program you use for viewing and/or editing your images. Look for instructions for 'optimizing' images for the web or for 'resizing'. (The software which came with your camera almost certainly has the appropriate functions).
If you haven't got any appropriate software you'll need to get hold of something suitable. If you're serious about your photography you ought to consider buying one of the versions of Adobe Photoshop. (It's certainly not cheap but it's great software). If you want a freebie, download Irfanview or The Gimp:
http://www.irfanview.com/
http://www.gimp.org/
(Irfanview's my preferred program for most graphics functions but The Gimpis possibly better for optimizing images for e-mail).
I'll repeat, however, that it's extremely unlikely that your camera wasn't supplied with suitable software. Check the help