How can I connect my Mac to a network server? As in windows yo go to My Computer/ connect to a network unit then you select a letter and write the name of the server you want to be connected to

To connect to a network server, in the Finder: Go->Connect to Server... (or press Command-K).

To have this connection always active, see this: https://www.tekrevue.com/tip/automatically-connect-network-drive/

There is no "letter" in OS X. That's a very old hangover from DOS, mapping devices and volumes to labelled letters.

If you are connected to your company's network, then you should see the available network volumes listed in the Finder, either under "My Computer > Network", or with Command K to Connect to a Server.

You can make an alias to the volume and put it in the Dock, or leave it on the Desktop, or put it somewhere else, and next time you want to connect on it, just double-click on it. You can also add the server address to your "favourites" in the Connect to Server dialog.

If you are working remotely then you are unlikely to see available servers automatically listed at the left of a window in the Finder. (This uses a feature called 'Bonjour' that normally only works locally.) Similarly the browse network or Go To Network options only list servers visible via Bonjour.

What you can do is use the 'Connect to Server' option from the Go menu in the Finder. In this dialog box you can then type the address of the server but you need to use the correct format.

In Windows you might type the following to access a server

\\Servername\Sharename

However the above only works on Windows, you need to use the Mac equivalent of the above, which might look like the following.

smb://servername.domain.com/Sharename

or

afp://servername.domain.com/Sharename

Or if your network is not remotely providing DNS information you might have to use the numeric address of the server like the following.

smb://192.168.1.10/Sharename

or

afp://192.168.1.10/Sharename

Note: SMB is the standard Windows file server system, AFP is the standard Mac file server system. If your company uses a Windows server - which I suspect will be the case you are likely going to need to use the smb:// versions I listed above.