How do I know if my Mac is infected with a malware?

It shouldn't be if you don't do dumb things.

Have you ever installed any software that did not come from the App Store or directly from the developer's website?

If you didn't, then you don't have malware.

If you did, then it is possible you installed Adware.

EtreCheck does a good job identifying suspect software. http://www.etresoft.com/etrecheck

If you think that it may be, you can use the following to check. It is one of the few such utilities that has a decent reputation: https://www.malwarebytes.com/mac/

I don't really have a clue. I'm new in the apple world.

Please see the articles below.

Effective defenses against malware and other threats:

https://discussions.apple.com/docs/DOC-8841

If you're checking for Mac OS X malware, click here.

If you're checking for Windows malware, use ClamXav or a Windows tool.

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