Your startup drive is the Macintosh hard drive - and in particular you'll need to look in your own account drive space (your Documents, Downloads, Pictures etc folders) and/or Applications folder.
A temporary fix is to restart your computer, this should set the temporary files back to zero and buy you a little time. You may want to download a program that shows you what files you have and where they are located. Omni Disksweeper is free and pretty good, I like DaisyDisk which happens to be on sale at the Mac App Store right now.
If you have lots of media files (video, music, and digital photos) they are the files that take up the most space. You'll probably want to purchase an external drive if you don't own one to transfer your files to. And don't forget to empty the trash once you've put things into the trash can. Putting files into the trash doesn't automatically delete them.