How to activate Google now voice commands from the lock screen like siri?

I just switched from an iPhone, and I want my android to have the same behavior as iOS where you hold the home button, and siri starts listening. With my android, I have to wake up and unlock the device, then hold the home button to get to the google now screen, then I have to either say "okay google", or hit the microphone button. This process takes way longer and makes voice commands in the car unsafe, which is primarily what I used siri for Please any help would be appreciated