Handbrake is a free open source video transcoder which you can use for this purpose.
After downloading and installing Handbrake, open the program and follow these steps to compress your video.
When Handbrake opens, press Source:
Choose where you would like the compressed file to go by pressing Browse:
Choose the output format, MP4 works nicely:
Choose the quality you want to output in order to reduce the file size below, in this case I'll reduce by around half, so I put the slider half way:
Once everything's ready, press Start and the file will be compressed: