I agree about downloading the FREE MalwareBytes anti-malware app.
You should run this once a week or so.
Contrary to the "prompts" to upgrade to the paid version, YOU DO NOT NEED TO DO THIS.
It will run in "free mode" forever.
Re Adobe flash:
I've had it installed for years and years and it has NEVER caused me any trouble.
You do have to keep it updated.
Clicking the following URL will always download the latest version directly from Adobe:
https://fpdownload.macromedia.com/pub/flashplayer/latest/help/install_flash_player_osx.dmg
I have it set up to NOT install updates automatically.
In fact, I don't permit ANY software on my computers to be "updated automatically", not even Apple's. I'll do the updates myself, thank you very much.
You should NEVER click on a link in a browser that prompts you to update flash.
Instead, open the flash pref pane, click the "updates" pane, and click the "check now" button.
And again -- if it says an update is available -- use the link I provided above.