If you have Photoshop, take the ruler tool and make with it a line parallel to the horizon. Then go to picture/rotate/manually and you have the exact number already filled in. just press OK.
If you don't have this feature and have to try, don't rotate several times, because each time the pixels are rearranged and each time information is lost. After three times you'll have a totally blurry picture. Rather try out an angle, and if it was too much, undo and try a smaller, and so on, until you have it.