It can be the first thing you teach them whilst building up a routine. I use these gestures in my class and children respond well. Of course it is optional, you might want to try something different. It's up to you. If you feel like, write me a comment how it works for you.
You can find this song on Super Simple Songs CD 1 Track 1.
Hello Song
Hello! Hello! (wave your right hand)
Hello! Hello! (wave your left hand)
Come in! (make 'come here!' gesture with your arms)
Let's sing!
Let's play!
Hello! Hello! (wave your right hand)
Hello! Hello! (wave your left hand)
Knock! Knock! (gesture knocking on an imaginary door in front of you)
Knock!
Com'on in! (make 'come here!' gesture)
Let's sing (make a circle movement with your finger, pointing around in the classroom)
and play!
And learn together!
Knock! Knock! (gesture knocking on an imaginary door in front of you)