My start up disk is full, how do I erase unwanted files to free up space?

Well, it depends which files you do not need. You can safely erase things in your User folder (things you put there). You did not furnish enough basic information to give spcific help - we'd need your Mac model, and exact OS version.

If you have a lot of videos, music, and apps, we can post links on how to move those permanently to an external drive; here is the latest info which may or may not be the right one for your OS:

http://support.apple.com/kb/PH2302

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

http://support.apple.com/kb/HT1449

Once the libraries are safely on an external drive, you can delete them on your hard drive.

Please note: you need an absolute minimum of 10 - 15 GB of hard drive space available at all times for the OS to operate properly - more is better.

See Here...

what-to-do-when-your-hard-drive-is-full.html

Also have a look at these 2 links...

> Where did my Disk Space go?

And Here > The Storage Display