how do i clean my start up disk and free up space

Download OmniDiskSweeper (http://www.omnigroup.com/products/omnidisksweeper/) to help you identify large files that you can safely delete.

Make a backup of your system first, and remember to be careful when deleting anything outside of your Home folder, or inside your Library folder.

I tried downloading it from the link provided and it opens a microsoft word window after I download it. I can't open the application

I have a iMac and desparetly need some freed up space. I have emptied trash for iPhoto, iMail, cleared extra disk images, checked for old printers, and then emptied my trash bin on the desktop. I have 616MB. Most of my space is taken up by photos, video and music that I would like to organize for our family. I have just started in with iMovie and will be working with many of those items, but due to the excessive amount of them, can I can move to an external hard drive and work on only one year at a time? (We have 7 years of digital overload to go through.) Is it easy to work with files once they are moved to an external hard drive? What would the best steps to take from here? Thanks in advance for any help you can provide!