Have the following questions. Read on for Answers.

I have a Receiver that my two big front speakers, center, and sub are connected through. I want to be able to stream sonos through this system.

I have an existing home audio system. I need some help to decide on what to get to upgrade my system to be Sonos enabled.

I want to be able to stream music from the internet through apps like iTunes, Apple Music, Spotify, Amazon Music, sound cloud, Google Play Music to my existing Audio System.

The answer to these questions is to Get a Sonos CONNECT.

With the CONNECT to have the content from your receiver/amplifier play on your SONOS network you can plug the output of your receiver/amplifier into the line-in of the CONNECT, in reverse to have your receiver/amplifier play what is on your SONOS network you would plug the output of the CONNECT into the line-in of your receiver/amplifier.

For the output on the CONNECT you can use either analog which is a standard RCA audio cable or you can use a digital connection which is through a TOSLINK® optical audio cable or a digital coaxial cable.

The CONNECT can run both sources at the same time. If you want everything on your SONOS network to be in sync with your receiver/amplifier this will come in handy as you can be receiving an input through the CONNECT which will be processed and then fed to the SONOS network and to your receiver/amplifier at the same time from the CONNECT.

Here are some links to help get you started:
Using Line-In on a SONOS player.
Using Line-Out on a CONNECT.

Alternatively you could use a CONNECT:AMP to replace your existing receiver/amplifier and have the necessary sources connected to a switcher and then that connected directly to the CONNECT:AMP with your speakers also powered from the CONNECT:AMP.