Is this a brand new amp or are you trying to connect an amp you’ve already been using to a new wifi?

If it’s brand new, plug in the amp, download the Sonos app, and follow direction for settting up a new system or device.  If it’s a new WiFi,  then follow these steps.

Thanks

I’ll try that when I get home. 
Do I need to pair amp with WiFi or just use the app on phone ?

You will use the app as a controller to connect the amp to wifi.  Once connected, the amp can play audio on it’s own without using the app.