Ok, here goes.
Once your photo is open, go under the view menu and make sure Extras is turned on, and then under Show in the View menu (just below extras), choose Grid.
You should have a non printing grid of squares over the entire photo. Grab the bottom right corner of the image window and drag outward to reveal the grey canvas area around your photo.
Press Control-A or go under into the drop down menu to Select the entire photo and then bring up the Free Transform bounding box around the photo. (Keyboard shortcut. Control-T). Move your cursor outside the bounding box and click and drag upward or downward to rotate your image (using the grid as a straight edge to align your image). If one of the horizontal grid lines isn't close enough to a part of your image that's supposed to be straight, just move your cursor inside the bounding box, and then use the Up/Down Arrow keys on your keyboard to temporarily nudge your photo up/down until it reaches a grid line.
Tip: If you want more control of your rotation (and this is particularly helpful when your trying to align to a grid, rather than just "eyeing it"), try this: While you have Free Transform in place, go up to the Options Bar and click once inside the rotation field. Then use the up/down arrow on your keyboard to rotate your photo in 1/10" increments, giving you maximum control
Go back under the View menu, under Show, and choose Grid to remove the grid. After you remove the grid, you'll notice that there are white canvas areas visible in the corners of your image, so you'll have to crop the image to hide these from view.
Select the Crop tool and then drag out a border that will crop your image so that none of the white areas are visible.
This is taken pretty much word for word from my Photoshop book.
I have tried the technique and it works pretty damn good, however, for some reason, the shortcuts don't work for me. But all the functions are in the drop down menus.
Hope it helps. Any questions, please feel free to ask.
Without struggle, there is no progress. (Frederick Douglass)