I'm trying to create a custom arrowhead in illustrator CC but can't seem to find out how.
Here is an Adobe Illustrator Help file that gives a method for adding custom arrowheads to Illustrator CC:
Look at the end of the file, in the section "Customize arrowheads" where it says:
To define custom arrowheads, open the Arrowheads.ai file, which is located under ShowPackageContent\Required\Resources\\ (for Mac) and \Support Files\Required\Resources\\ (for Windows). Follow the instructions in the file to create custom arrowheads.
Place the updated Arrowheads.ai file at: \Plug-ins\ and avoid replacing the existing Arrowheads.ai file.
The "tv.adobe.com" link given above is not relevant; it tells you how to make a pattern-brush arrow, but those don't behave like real arrows.