How do I get the file size down?

I took a video that I'd like to post but i have to edit it first, I can't edit it in a down-loadable software because I can't download anything without a password to my dads account on my laptop. I've got no video editing tools on my laptop so I have to do it online well I found a place but my videos are HD videos and they are pretty big files the website doesn't allow anything over like 600 mb but my videos are huge! Is there any way to get the file size down without retaking the video? please help!