imac won't boot because disk is full

About a year ago, my mac was giving me messages that the memory was almost full, before I got a chance to clear space...it stopped booting. I just pulled it out of the attic to try and re-boot and all of the easy steps on forums aren't working. Disk Utility won't repair, and I can't clear space if it won't boot. Tried to get it to boot in safe mode, but it crashes before completing the boot. I just need to get in long enough to clear some space and re-boot. I also tried single user mode, but it won't give me a prompt. Any help is appreciated.