Guides can be really helpful when composing your artwork in Photoshop. Many users often find themselves tediously trying to place a guide at the exact center of a Photoshop document by dragging the guide out of the ruler. Even holding the Shift key down to have the guides snap to the tick marks on the ruler, it can be a pain. There’s an easier way to do it. 
Go to View>New Guide and choose either a horizontal or vertical Orientation by checking the radio buttons. Then type 50% in the position box. Repeat the process for the other Orientation. You’ll now have guides at the exact horizontal and vertical in your document. Now that you’ve created your guides, you want them to stay centered. But if your guides are locked into place and you crop or otherwise resize your image, the guides remain wherever they are, including off the image completely depending on where they are located. To have them remain at the 50% mark no matter how you crop or resize, simply unlock your guides by hitting Command + Option + ; (or visit the View menu and unlock them manually). Alternatively, if you really want the guides to stay in place, lock the guides before your crop or resize. So that vertical guide you placed at the one inch mark will remain there after you resize your image (unless of course you resize your image to smaller than one inch).
Neat was struggling hard to place the guide. Thanq man