Straightening (in PS)
Straightening an image in Photoshop is fairly straight forward. Other editors also have this feature.
Click the crop tool (circled red).
This will give you the menu at the top of this screenshot.
If anything is selected (e.g. a ratio from your last crop) clear it (hit the clear button), [delete cropped pixels] should be unchecked as should [content aware].
You can see the outline of an ideal crop starting point around this image.
We need to decide what is straight. Vertically you could go from the arch to the peak of the brickwork but the photographer was a little off centre when taking the image and I don’t think that works well. An alternative is the bottom of the door (see the red lines for possible alignments).
So click the straighten button (middle of the top menu), then right-click-and-hold the bottom left hand side of the door. Drag your mouse across to the right hand side of the door and when you feel the line is EXACTY level let go of the mouse button.
If you don’t like the result press ESC and try again. If you are happy press ENTER to commit the change. The same works for vertical straightening right-click-and-hold the mouse button at the top of a vertical line, drag down to the bottom of that line and release the mouse button when you are sure.
The more you skew the crop the more you will lose from the corners. If you select (tick) content-aware BEFORE you crop Photoshop will try and fill the cropped corners with what it thinks should be there. Sometimes more successfully than others!
If you have delete cropped pixels UNticked Photoshop will remember what was there and later you can return and resize the crop and regain some “lost” data. If you select (tick) this option then when you press ENTER PS will delete the pixels outside the crop and they are forever gone (except on your original which is safe somewhere else :-) )