If your camera came with some software (which almost every camera does), there should be an option which lets you do what you require. (Look for 'resize' or 'compress' in the help files).
Otherwise, you'll need to use some other appropriate software. If you're lucky enough to have Photoshop, then File > Save for Web is a superb option which lets you preview the quality of compressed images as well as telling you the new file sizes and estimated upload times.
Alternatively, get hold of the excellent free image viewing and manipulation software called IrfanView:
http://www.irfanview.com/
Use the Image > Resize/Resample option. (A bit of experimentation might be required to get the file size you require. Try 800 x 600 or 640 x 480).
Other people sometimes recommend Picasa. I've not tried it but, since it's free, you might want to take a look:
http://picasa.google.com/
Chris