The first step is to switch on the guides and rulers. Either use the keyboard shortcut Ctrl+R or go to the View menu and select Rulers. You'll see a scale appear around your photo.
We need a horizontal guide line to help get the horizon level. To get this, click anywhere on the horizontal ruler at the top of the image and drag your mouse downwards. A blue line will appear which you can drag down onto the image using the mouse. The Move tool (V) is best for this.
Drag the guide down and position it across the middle of the horizon line.