When I start my MacBook Pro, I get a message that says "your startup disk is full you need more space by deleting files." It then gives a box to check ok. After about 30 seconds, it fades to a gray screen and I can't do anything. I've tried everything!

1. If you have a bootable backup, then boot from it in order to clear files from the disk.
2. If you have another Mac you can connect them via Firewire cable. Boot yours into Target Disk Mode, then boot the other one normally. You can access the disk from the other computer.
3. Replace the drive with a new, larger drive and reinstall OS X onto it. Put the current drive into an external enclosure and attempt to access it as an external drive.

First -- do the target disk backup. Somehow get a backup.

You might try booting to Single user mode: http://support.apple.com/kb/ht1492 and removing the files in the command line. When you put files in the trash, the space is still used up until the trash is emptied.

using rm command removes it totally. Just be very careful because you remove the wrong file, and you have to relad the system