System Preferences > Displays > Arrangement

From here you should see the Arrangement diagram of all your displays. Under that, you will see the "mirror" option. Checked to mirror, unchecked to extend.

Your post is a bit confusing to me, It kind of seems as tho you want to have an extended desktop so you can put files/folders over on that monitor so it's less cluttered on your main display. It also seems that you just want to see the exact same files/folders that are on your main display, but over on your newly installed 2nd monitor (That was be the mirror function).