On the machine that you are connecting from, while in the Finder, either click on the ‘Go’ menu in the Finder or 'Command ⌘ + K' keys to bring up the ‘Connect to Server’ dialog box. 

You can also drag the Screen Sharing.app to your Dock as an alias and launch that every time. Once opened right-click Screen Sharing in the dock, choose options and select ‘keep in dock’

Whichever method you use to connect, the address you enter is the same: vnc://ipaddress

When the connection is made you will be prompted to authenticate as an admin user account on the machine you are logging into.

Enter the correct details and you will then see and control the remote computer on your screen.