You should right-click on the desktop and in screen settings (I'm not on a W10 PC atm, so I can't give you exact instructions on this). When you see a picture with 2 screens you will know you are in the right settings. Under the picture you have the options on what you want the monitors to do, right now it should be "mirror picture from screen 1" or sth like that. For what you want, you need to choose "extend desktop to this screen". Also for multiple monitors setups I suggest using some software like DisplayFusion which is avaliable for free or in PRO version.