AntiVirus

The best defense against viruses is knowledge and common sense!

Antiviruses are software suites which keep a constant eye on your computer and try and prevent and assist you if malware is found.

Windows Defender (The built in antivirus for Windows 7+) has come along way in the past years and for general usage should be enough, Have realtime protection enabled, with SmartScreen enabled.

To keep a long discussion short there is debate as to whether or not an active antivirus actually makes it easier for viruses in modern times. If youre interested in reading more detailed on this topic consider reading TheRegister.co.uk's Article. They are also the biggest cause of a low grade consumer machine to be slowed down (as they have a heavy active scanning components)

If you're looking for a well respected antivirus you can consider buying Malwarebytes Premium/BitDefender (Feel free to look into reviews yourself, but this was our findings as of May 2017)

For Mac

Macs/OSX are built differently and a smaller market meaning there is less malware created for them, this does not mean that you are perfectly safe! With smart browsing habits an active antivirus is not needed.

Again if youre looking for a well respected antivirus check out AVG antivirus and BitDefender, these two were the highest rated/sucessful antivirus for Mac/OSX as of May 2017.

For Linux

Only downloading legitimate software packages and safe browsing habits will generally stop any malware.

Checking for rootkits can be done by installing and running CHKRootkit & RKhunter:

sudo apt-get install chkrootkit
sudo apt-get install rkhunter

Antivirus wise the OpenSource ClamAV is your best bet https://www.clamav.net/

 sudo apt-get install clamav

again as previously mentioned, the "apt-get" command is available for debian based systems, as this is a beginner guide this should cover most users (Linux Mint/Ubuntu etc)