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

In Google Now, click the menu icon on the left of the search bar (it looks like a hamburger).

Then choose Settings -> Voice -> "OK Google" Detection.

(Click image to enlarge)

'OK Google' Detection screen

Flip the three switches on. When you switch on the second one, the app will ask you to say "Okay Google" three times to train the voice model. Flipping the third switch on will prompt you for the password to your phone.

Now, when you say "Okay Google", the screen will be unlocked.

In Android, by default, you can't have access to Google Now when the phone is locked (maybe for security reasons), but there could be a lock screen widget for that. You can search for that.