It's a bit difficult to answer your question without knowing which software you've got available. Your camera almost certainly came with some suitable software and a quick look at the help files should tell you what to do.
Otherwise . . .
If you're lucky enough to have Photoshop, then 'File > Save for Web' is a superb function because it allows you to preview the image quality and estimates how long the file will take to upload.
Irishlyn mentions Irfanview. This program should be regarded as essential for almost everyone. It's a superb image viewing program with lots of advanced options for image manipulation. (For your requirements, select Image > Resize > 640 x 480 > OK). You can download Irfanview, for free, here:
http://www.irfanview.com/
Irishlyn also mentions Picasa. I've not used it but it should do the job you require. (Note, however, that it only runs on the dreaded Windows XP, which I refuse to have on my PC, or Windows 2000):
http://picasa.google.co.uk/
While I'm happy to support Ratter14's suggestion of using Photoshop, I have to disagree about using WinZip (or any similar program). Your pictures are probably already compressed (i.e. they're in jpeg format rather than bitmaps). Zipping the files won't be able to provide much additional compression. (Typically, the file sizes would be reduced by 5% to 10% at most). So, using WinZip won't significantly reduce your upload times.
Chris