My startup disk is full

Get an external storage device. If you don't have a lot of money, get the biggest USB flash drive you can afford; otherwise get a USB or network drive at least as big as the drive in your computer. If you can get a big drive, run Time Machine to back up the computer to it.

The following directions assume you are unable to do a full backup. If you did the backup, you may ignore the copy instructions, and just do the deletes. Next, open your (username) Downloads folder, and dump everything you don't recognize. Go to (username) Documents, and copy everything you want to keep, but don't expect to use for a while, to the flash drive. Delete all of those items after they are copied. Delete anything you are sure you don't need to keep. Repeat this process in the (username) Movies folder. Note: if you have movies stored on the laptop, this is where you will get the greatest space gain. The Pictures folder is the next one to clean out - picture files are typically large, although not nearly the size of movies. Finally, open your web browser(s) and empty the cache (for Safari, the keyboard shortcut is command-option-E, otherwise look in the application-name menu between the Apple and File menus).

It's a bit of a chore, but not as hard as cleaning out a garage, and much cheaper and easier than taking apart the computer to put in a bigger drive, especially if you have one of the new machines with a flash drive soldered to the logic board.