It's the circular button at the bottom of the phone's screen. Doing so will invoke the Siri menu after a second or so; Siri will open in "listen" mode and await your instructions.  If your iPhone uses AssistiveTouch for a broken Home button, tap the on-screen AssistiveTouch square and then tap Siri (or press and hold the Home icon). You can also say "Hey Siri" out-loud if you enabled the "Hey Siri" function. Once you see a multicolored line appear, you're free to begin talking to Siri. While Siri can handle most iOS-internal requests (e.g., calling a friend), Siri will search the internet for an answer to more complicated questions. It's in the bottom-left corner of Siri's screen. Doing so will bring up a list of apps with which Siri can interface and a brief description of how to use them. Doing so will close Siri. You can also say "Goodbye" if Siri is in listen mode.

Summary:
Press and hold your iPhone's Home button. Wait for a rainbow line to appear at the bottom of the screen. Ask or tell Siri something. Tap ?. Press the Home button again.