I am getting a message that says my start up disk is almost full and I need to delete some files. What/where is my "start up disk"?

The startup disk is your MacBook Air's internal storage, often named Macintosh HD. That message means you need to remove material from your internal drive. An easy way is to get an external hard drive and move big stuff like videos, tunes, photos to the external drive so have free internal space.

Freeing Up Space on The Hard Drive

1. See Lion/Mountain Lion/Mavericks' Storage Display.

2. You can remove data from your Home folder except for the /Home/Library/ folder.

3. Visit The XLab FAQs and read the FAQ on freeing up space on your hard drive.

4. Also see Freeing space on your Mac OS X startup disk.

5. See Where did my Disk Space go?.

6. See The Storage Display.

You must Empty the Trash in order to recover the space they occupied on the hard drive.

You should consider replacing the drive with a larger one. Check out OWC for drives, tutorials, and toolkits.

Try using OmniDiskSweeper 1.8 or GrandPerspective to search your drive for large files and where they are located.

As Ralph has told you your startup drive is the one usually named, "Macintosh HD." It is the internal drive which may be an HDD or SSD depending on which model of MBA you have.