Open the photo in the Photos app
Click Edit
Click Crop
Drag the corners or edges to select the area you want to keep
Move the image inside the crop frame if needed
Choose an aspect ratio if desired
Click Done
Open the photo in Preview
Click the Markup toolbar button
Click the Crop tool
Drag to select the area you want to keep
Click Crop
Open the photo in the Mac screenshot editor if you just took a screenshot
Click the crop icon
Adjust the selection
Click Done or Save
Use the Photos app keyboard shortcut Command + Return to enter edit mode
Use the crop controls to adjust the image
Save your changes when finished