My computer keeps telling me that my startup disk is full. how do i empty it? Also, i want to add all of my music back onto my computer from my external hard drive but it won't allow me because it says that my startup disk does not have enough space.

If you have a lot of video podcasts in your iTunes they take a lot of space on your hard drive. delete the one you don't want and make sure the empty the trash can.
Try a piece of software called "Disk Inventory X"
Disk Inventory X is a disk usage utility for Mac OS X 10.3 (and later). It shows the sizes of files and folders in a special graphical way called "treemaps".

If you've ever wondered where all your disk space has gone, Disk Inventory X will help you to answer this question.

When your Mac has been used for a long time. If you do not clean the junk files on Mac often, your Mac's startup disk space will be filled. At this time, you only need to clean up your Mac to solve this problem. You can clean up your startup disk space based on the following ten aspects.

1. Remove old iOS backups
2. Remove iOS software updates
3. Remove unnecessary applications
4. Clean up the cache file on Mac
5. Delete unnecessary language packs
6. Delete the old iTunes library backup
7. Clean up your photo library
8. Clean up your downloads, movies, and music folders
9. Clean up your desktop
10. Empty out the trash can

If you find it difficult to clean these ones manually, you can download some Mac cleanup tools (such as my recently used iMyMac's Mac Cleaner software) to help you clean up these things.
I hope my answer can help you.