I'm using Ubuntu Server 16.04.4 LTS. I didn't install any antivirus program there. Is that necessary to install any antivirus program there?
There are viruses for most all platforms (the first worm was for DEC VAX) , they are just more common on Windows. Different platforms are more secure than others, but a virus can typically gain user level security, which is often good enough, on most platforms. You can actually run Windows without a virus scanner if you keep it patched and are really careful.
Differences from Windows to Linux for viruses: On Linux it is harder for the virus to get root (or system) level access. But it could probably still access your address book or saved passwords in Firefox (user level access).
Strategy: If you are really careful and know what you are doing you can get away without a scanner. A good strategy is to have your scanner just scan downloads or if you are bringing a file from another computer on a USB drive or floppy disk (assuming you have one). If you want you can have it do regular full system scans too. It is all about how paranoid you are, and how likely you are to get a virus.
For your laptop I would suggest only having the scanner scan downloads and when you are brining files from another computer. Having it do full system scans can be a battery drain.
A very important element of a virus scanner is having it updated with new signatures, so pick the package that has the best signature updates, and that works best for how you want to use it.