Can I charge my headphones with the MacBook 12" charger? Can I charge my headphones with the MacBook 12" charger or should I use it for charging the MacBook only?

The headphones are Plantronics BACKBEAT SENSE which have a USB-Micro charging port.

I believe I need to buy a cable USB-C to Micro-USB, if it would work.
A USB charger is universally compatible with devices that use the USB interface to recharge.  An Apple MacBook USB charger will charge any device including Android, Microsoft, or no-name type devices - including your Plantronics headset.

Apple provides you a USB-C port as shown in the image (below left) .  You just need a cable that's compatible with your device and in your case, a USB-C to USB micro cable (right)




As long as you have the right cable, you can recharge anything so long as you don't exceed the power rating on your adapter.

This means that if your headphones require only 5W to charge, then this adapter will work.  If your Android tablet requires 10W it will charge.  However, if your friend brings his shiny new MacBook Pro which requires 65W, it will either charge very slowly or not at all.

You can always use/charge devices that are less-than or equal to the wattage rating of the charger, but not vice versa.

As for picking adapters/cables, always buy a quality product.  You can rest assured that Apple is a quality brand (though expensive).  As for cables, stick with reputable manufacturers from reputable sellers.  Saving a few dollars on poorly made adapters can cost you dearly in the long run.  See this video for what happens.