how do i make my movies file size smaller?

i put 15 min of video on my computer from my dital camcorder and the video came out being 4 gigs!!!!

i really need to try and shrink the file size for obvious reasons...

the computer saves the movies as an AVI file, and from what i understand i need to re-encode the AVI file to an MPEG......i just need help, so give me advise