Pro Tip #1: We recommend taking the following measurements with a tape measure. (Accept no substitute, including anything you print from online!)
Pro Tip #2: We HIGHLY recommend taking all of these measurements with a nylon wig cap on so that all of your existing biohair is nicely compressed, as it would be under the cap of your wig. This will also give you the added benefit of providing a "guide" for the tape measure at the perimeter of your hair line.
When shopping for a wig, the most important of these measurements will generally be the first one. Wig sizing is done by circumference most of the time.