How can I install Avast on a brand new iMac securely?

I currently have a PC (Windows 7) machine with Avast Premier .
I am concerned about connecting my new machine to the internet prior to setting up the virus protection?
I have downloaded the file ‘avast_free_mac_security_online.dmg’ onto a flash drive.
Any advice on what to do next would be most welcome. (I am a complete ‘Newbie’ to a Mac)

See: https://www.avast.com/faq.php?article=AVKB69

Many thanks for this information Asyn, this is a great help!

You’re welcome.