Replying to FInal Cut Pro Compression/De-interlacing

Could someone help me out and explain to me the best way to compress a video? Right now I have just been exporting to quicktime, then opening in Mpeg-2 and compressing in there. I can typically retain the quality of the video, but it never compress very much. Is it even possible to compress a 1gb movie into about 100 mb?
Also I know the de-interlace tool in FCP isn't the best, is there anything I can do with out downloading plug ins? (working on schools computers) And what are the best plug ins so I can download them for my computer..