I've taken a screenshot, how do I edit it? I'm used to taking screenshots in Windows and then editing them before posting them (with MSPaint)

How can I edit a simple image quickly on OSX?  

I took a picture of a P.O. and I want to remove any PII information
There is a default application named Preview in OS-X for editing images.

If you created any screen shots, just open that screenshot image in Preview application and you can see the edit toolbar to edit the image.