When you say center, do you mean making the webpage even? The best thing in photoshop is to turn on the grids and use the guides.
For grids, go to View -->Show-->Grid This will automatically show your webpage in even blocks, sometimes they are uneven you might have to adjust the canvas size or trim.
The guides are essential if you want to place layers on your webpage and have them even with the rest of the design (or if you want to center images). Just go to View-->New Guide choose vertical or horizontal (just drag the guides where you want according to the grid). The guides snap to the grids automatically so you shouldnt have a problem properly centering with this method.
I realize you're just trying to help and I apologize if I sounded brash. However, again the advice you are giving is not very good. The <center> tag is deprecated and should not be used! You can read more about deprecated tags, including the <center> tag here.
Also I dont recommend youtube for ANYTHING in relation to learning web design and/or coding. You really dont know WHAT you're getting watching those videos. Would you want your surgeon watching youtube to learn how to perform a new procedure that you are getting ready to undergo?..lol Youtube is rather like wikipedia....cant be trusted in cases like this.