Hi Krisneth:
In your screen saver control panel, select a screen saver. Then, under the activation tab, check whether you want it to ask for a password. Then, under Hot Corners, put a tickmark in a certain corner (I use lower right). Any corner that has a tickmark will activate the screen saver if the mouse is dragged there and left for a moment, any corner that has a minus-mark will disable the screen-saver if you leave the mouse there (Sometimes handy).
That should do it ...