How-to manage network connections.

If not set during installation, become root typing su either in a
console or in graphical terminal like mate-terminal and type the root
password, then type the command 'netconfig' (without the quotes).
You will be first asked for the hostname of your machine. you can choose
any name, but it has to be a single word.
Then you are then asked the domain name. If the machine is not a server
you can choose any one.
Then you will be asked how your machine connects to the network. Unless
you have a good reason not to and you know what you are doing, keep
the default NetworkManager to autoconfigure the network.

For the wired connections, that's enough to be connected on next boot.

To configure the wireless connections, two commands are available using
the command line.
 
Again, you can type the commands directly on the console or in a
graphical terminal like mate-terminal. Type them as regular user not as
root.

These commands are nmtui (dialog boxes) or nmcli (pure command line),
both accessible.

Using nmtui, you can navigate with the Tab and arrow keys.
You will be proposed three options:
Edit a connection
Activate a connection
Set system hostname

If no wireless connection has been set yet choose Activate a connection.
You will be proposed from top to bottom the wired connections (if any)
then accessible wireless networks. Navigate with the down and up arrow
keys to select the network you want, then just type Enter: you will
probably be asked the password or encryption key. Type it, use the Tab
key to confirm, then exit and you are done.

nmcli brings more possibilities, but needs that you first know how to
use it so first type 'nmcli --help' and know more 'man nmcli' (without
the quotes).

Then in graphical mode only, you can use the application
nm-connection-editor.