This is just a general guide on how to keep your computer secure. It is centralized around MACs, but can be used for other Items.
Password Security
This is a fundamental part of computer security. Without a secure password, your computer is open for the taking. Below is a password guide I recommend you follow. (Please note that if you don't have any sensitive information going on that computer that this may not be as important)
Change your password every 3-6 months
Use Different passwords for different things (make sure you remember them)
Don't write it on a sticky note or text document
Use a minimum of 12 characters
* Phrases Help (Example Crypt0.W4ll3t.1s.53xy
Storing on external media is also a good idea as you can disconnect it from the network when needed and won't have to worry about risk of virus
Updates
Basically, make sure your computer, antivirus, and anything else is up-to-date and set to update when needed
Always use Antivirus
Windows Defender I hear some people say "It's good", but in my view that won't cut it. Hackers plan on you to have it and right code around it.
*I recommend if you don't have any to either purchase webroot or use (avast, bit defender, super anti spyware, ext... that are free)
* Be warned the paid software have more features, but don't necessarily mean they are better
Use a VM
If you do get a virus, one handy tool is to work within a VM. A VM is basically a virtual computer within your computer. If you do get a virus, all you have to do is delete the VM and re-create the VM and won't have to worry about infection.
VMware and virtual box are good free software
*If you use Linux or MAC, you have less chance of getting a virus
What to do if you think you got a virus
The safest solution would be to re-image the computer (re-installing the operating system). If you don't want to do that here are some good tools you can use. I recommend when you use these to boot into safe mode (google if you don't know what that is)
Super AntiSypware
TDS Killer
RR Killer (Run first, it kills the virus but doesn't get rid of it)
Combo Fix (Very Thorough, but can damage computer)
Malwarebytes
Avast
Bit Defender
Complete Internet Repair
ADW Cleaner (Good for adware, but can get glitchy)