To straighten a photo you have to rotate it.
Measuring necessary angle
To measure the angle use the "Ruler Tool" 
. Drag a line along the horizon and read the Slope value, or drag a line along vertical element and read the tilt value:
Rotating the image
To rotate the image choose "Edit" > "Rotate" > "Arbitrary". Enter the rotation angle in degrees.
CW rotates the image clock wise (top of the image to the right)
CCW rotates the image counter-clock wise (top of the image to the left)
Select Crop option to remove blank background.
 Original |
 No crop |
 With Crop |