Open the photo or image in Preview
Click the Markup toolbar button
Select the Rectangular Selection tool
Drag to select the area you want to keep
Click Tools
Click Crop
In Photos, open the image
Click Edit
Click Crop
Adjust the crop frame
Click Done
In Screenshot mode, press Shift + Command + 4
Drag to select the area you want to capture
Release the mouse or trackpad
In Pages, Keynote, or Numbers, select the image
Click the Format sidebar
Click Image
Use the crop controls
Press Return or click Done