Startup Disk is full

I'd recommend going over your disk and seeing what all is there. If you're not using programs/apps, delete them. If your disk is full of data/photos/songs, you might move them off to USB storage.

Step 1

The first step in dealing with a startup failure is to secure the data. If you want to preserve the contents of the startup drive, and you don't already have at least one current backup, you must try to back up now, before you do anything else. It may or may not be possible. If you don't care about the data that has changed since the last backup, you can skip this step.

There are several ways to back up a Mac that is unable to start. You need an external hard drive to hold the backup data.

a. Start up from the Recovery partition, or from a local Time Machine backup volume (option key at startup.) When the OS X Utilities screen appears, launch Disk Utility and follow the instructions in this support article, under “Instructions for backing up to an external hard disk via Disk Utility.” The article refers to starting up from a DVD, but the procedure in Recovery mode is the same. You don't need a DVD if you're running OS X 10.7 or later.

b. If Step 1a fails because of disk errors, and no other Mac is available, then you may be able to salvage some of your files by copying them in the Finder. If you already have an external drive with OS X installed, start up from it. Otherwise, if you have Internet access, follow the instructions on this page to prepare the external drive and install OS X on it. You'll use the Recovery installer, rather than downloading it from the App Store.

c. If you have access to a working Mac, and both it and the non-working Mac have FireWire or Thunderbolt ports, start the non-working Mac in target disk mode. Use the working Mac to copy the data to another drive. This technique won't work with USB, Ethernet, Wi-Fi, or Bluetooth.

d. If the internal drive of the non-working Mac is user-replaceable, remove it and mount it in an external enclosure or drive dock. Use another Mac to copy the data.

Step 2

You might be able to start up in safe mode even though you can't start up normally. Otherwise, start up from an external drive, or else use the technique in Step 1b, 1c, or 1d to mount the internal drive and delete some files. According to Apple documentation, you need at least 9 GB of available space on the startup volume (as shown in the Finder Info window) for normal operation.

@bartzkrieg, Please be very careful when suggesting users delete Applications - they can make an OS unstable if the wrong thing is deleted, it is too easy for a user to decide they don't need critical parts of the OS.

@sarafromthesticks, take a look at the 'More like this' at the bottom of this page for the general advice.

There is a user tip that covers this in depth but I can't find it (hopefully someone will post that here).

The general process is…

Be sure you have a backup before you begin.

Reboot

Empty your trash.

Look inside Applications that have their own trash (iPhoto, iMovie etc) empty those trashes too.

Move on to looking at where your disk space is used via tools like GrandPerspectiv, WhatSize or Omni Disk Sweeper.

http://grandperspectiv.sourceforge.net/

http://whatsizemac.com/

https://www.omnigroup.com/more

Once you have found any large files or folders you can move them to an external disk or delete them, please ask if you are unsure - certain files must be kept in correct locations to work, such as iTunes media or iMovie content.

Ideally I think you want at least 20GB free, if you have an older spinning disk (not an SSD) I would suggest more free space.

You're right, I wasn't being clear. I meant apps downloaded from the App Store, not the ones that came within the operating system. Or other parts of progams, such as Microsoft Office. Since I don't ever use Keynote, I've deleted it. But again, that's a program outside the OS.