how to clean my macbook air when startup disk is almost full

If the drive becomes too filled then you will no longer be able to empty the trash or restart the computer, shut down, etc. When this happens you will have to erase the drive in order to proceed. If you haven't gotten that far yet:

How to Free Up Space on The Hard Drive

1. You can remove data from your Home folder except for the /Home/Library/ folder.
2. Visit The XLab FAQs and read the FAQ on freeing up space on your hard drive.
3. Also, see Freeing space on your Mac OS X startup disk.
4. Free up storage space on your Mac.
5. See Where did my Disk Space go?.
6. Be sure to Empty the Trash to recover the space.
7. Replace the drive with a larger one. Check out OWC for drives, tutorials, and toolkits.
8. Use OmniDiskSweeper or GrandPerspective to search your drive for large files and where they are located.

If you are stuck.

Install Lion Through Yosemite from Scratch

1. Restart the computer and after the chime press and hold down the Command and R keys until the Apple logo appears. Wait for the Utility Menu to appear.
2. Select Disk Utility from the main menu and click on the Continue button.
3. After Disk Utility loads select the volume (this is the indented entry, usually Macintosh HD) from the side list. Click on the Erase tab in Disk Utility's main window.
4. Set the Format type to Mac OS Extended (Journaled.) Click on the Apply button
5. When the process has completed quit Disk Utility and return to the Utility Menu.
6. SelectReinstall OS X and click on the Continue button.

Install Sierra or El Capitan from Scratch

1. Restart the computer. Immediately after the chime hold down the CommandandRkeys until the Apple logo appears. When the Utility Menu appears:
2. Select Disk Utility from the Utility Menu and click on Continue button.
3. When Disk Utility loads select the volume (indented entry, usually Macintosh HD) from the Device list.
4. Click on the Erase icon in Disk Utility's main window. A panel will drop down.
5. Set the Format type to Mac OS Extended (Journaled.)
6. Click on the Apply button, then wait for the Done button to activate and click on it.
7. Quit Disk Utility and return to the Utility Menu.
8. Select Reinstall OS X and click on the Continue button.