Guides are horizontal and vertical lines that you can place in your document. The main purpose is to help you align objects inside your canvas. Guides are magnetically attracted and help you align objects precisely. To make your life easier use guides and you will stop looking for the center when you want to align objects or text.
Create Guides (method 1)
To add a single Guide go to View > New Guide and use the orientation options to place the guide in your desired position. Fo example, if your document is 800 pixels wide and you want to place the guide in the center, use Vertical Orientation and 400px position.
Create Guides (method 2)
Another way to create guides is when using rulers. To show the rulers around your canvas go to View > Rulers or press (Ctrl + R). To create guides, simply point your mouse over the guide and drag it inside your document. Go to View > Lock Guides to lock your guides position. To clear the guides go to View > Clear Guides. To change the color double-click on your guide and use the settings tab.