Hi the answers above are correct but i will try to make them easier for new ubuntu users.my ubuntu version is 18.04.1 Lts . to check youyr ubuntu version try to open your settings -> then go to details.
first step activate sharing on our desktop:
under settings ->select sharing and click on the large button screen sharing
you will have a popup windows under that windows make sure to select the option Allow connections to control my screen.
under , you have to kind of access method.I recommend to select require password and to specify a password that you will use to connect to your desktop.Please under networks section , enable the network shared between the two equipments.
second step connect to your desktop from your laptop ubuntu comes with Remmina (if not available under Activities -> search Remmina , please install it) open remmina .You will see a "plus (+)" button to create new connection , please click on it .Enter any value for field name , you can leave the field group empty , for protocol select (VNC -virtual network computing)
under Basic , for network enter the IP address of your desktop.
you can fill anything for the field user name. on the field user passsword enter the password defined when sharing screen on your lapTop.For resolution field you can leave "use client resolution" selected.As color depth select (True color 24bpp) and finally for quality you can select medium.
Save all your modifications . The created connection should appear on remmina windows just do a right click and select connect. You will see your desktop screen appearing.
remarks: your desktop firewall should be disable ( check it under your terminal with the command ufw status ) And be sure tne both equipments are connected through the same network . Hope this will be helpful for someone.