I have just tried installing Bitcoin Core recently as I was told its more secure than other online wallets, but i need some help regarding the following:-
- where to set the secret phrase

Do you mean your password? Or seed?
Bitcoin Core doesn't have a seed. You'll need to back up your wallet.dat file.
For password, go to Settings -> Encrypt wallet.
- how to know my private key

How to export your private key (ignore the importing to Omniwallet part)- My receiving address is it the same or does it change

It changes every new transaction.
- if my machine crashes how do i restore my funds (i know they are stored on the blockchain but how to have access to it with BTC Core??)
- what files do i need to backup and how to RESTORE it on a different PC or after PC reformat/crash ??...
Your wallet.dat file.
So the wallet.dat file that is in the installation directory is the only file i will need to restore access to my wallet in case my PC burns or worse ?...
I have also encrypted my wallet, but does that mean if my machine ever got hacked, i am 100% safe ?.. what to do for Maximum protection of my funds stored on my PC ?...