Please Help! My startup disc is full, and when I turn the computer on and log in then it warns me that my disc is full and then goes to a blank gray page. What can I do to delete files?

Go step by step and test.

1. Start up in Safe Mode.

http://support.apple.com/kb/PH11212

2. Empty Trash.

http://support.apple.com/kb/PH13806

3. Disk space / Time Machine ?/ Local Snapshots

Local backups

http://support.apple.com/kb/ht4878

4. Delete old iOS Devices Backup.

iTunes > Preferences > Devices

Highlight the old Backups , press “Delete Backup” and then “OK”.

http://support.apple.com/kb/HT4946?viewlocale=en_US&locale=en_US

5. Re-index Macintosh HD.

This will take a while. Wait until it is finished.

System Preferences > Spotlight > Privacy

http://support.apple.com/kb/ht2409

6.Try OmniDiskSweeper. This will give the storage size details of the items.

https://www.omnigroup.com/more

Select Macintosh HD and click “Sweep Selected Drive” at the bottom.

Delete the files yo don’t want to keep.

Be careful. Delete only the files that can be safely be deleted.

7. Move iTunes, iPhoto and iMovie media folders to an external drive.

iTunes

http://support.apple.com/en-us/HT201562

iPhoto

http://support.apple.com/kb/PH2506

iMovie

http://support.apple.com/kb/ph2289