Open the image in Photoshop
Select the layer in the Layers panel
Unlock the layer if it is locked
Choose the Object Selection Tool, Quick Selection Tool, or Magic Wand Tool
Select the subject you want to keep
Click Select and Mask
Refine the selection edges if needed
Set Output to Layer Mask or New Layer with Layer Mask
Click OK
If using a mask, invert it if necessary
Use the Brush Tool on the mask to clean up remaining areas
Save the file as PSD to keep the editable mask
Export as PNG if you want a transparent background