Startup Disk Full

I keep getting a pop-up saying my start up disk is full. I have tried deleting files one at a time to create space, but after much time spent I still get that pop-up message. I don't know how to proceed. IMPORTANT: I do NOT want any of my photos to delete! I'd really appreciate it if someone could advise me what to do. Isn't there some kind of clean-up app I could use? Because if I continue deleting files one at a time I'll be dead before I get finished. And I need most of the stuff I've saved. Thanks for any help.