What happens when you try exactly?
To move the mouse to a second monitor the displays need to be set to extend the desktop.
If you see the same thing on both monitors, and you see your mouse move at the same time on both, then you are mirroring the display instead of extending it.
Also in System Preferences, make sure the second monitor you are using is set on the proper side of the primary one, so the movement of the mouse is natural. i.e if the second monitor is to the right of the primary one, make sure the displays in System Preferences mimic that setup. Otherwise you may need to move the mouse to the opposite side of the monitor to get it to move onto the other monitor.
Read here for specifics:
Use multiple displays with your Mac - Apple Support