You'll have to decide which videos and music files you really need to have on your computer and which you can safely export to external media. Then transfer all unnecessary files to DVDs, memory sticks or an external hard drive.
Chris
PS: I've referred to video and music files because, for most home users, They're usually what consume most space on a hard drive. Photographs generally take up less space but, if you've got many thousands of them, you might also gain quite a bit of space by transferring them to external media. However unless you've been writing lots of really long novels, you won't gain much by transferring things like Word documents.