For kids who have never written poetry (or teachers who have never taught it), a great way to begin is with class poems. This is where each child writes one line from a starter provided by the teacher, and then all the lines are put together. The teacher should help the process along a little by making suggestions for small improvements. But remember, at this point the kids don't have much confidence in writing poems and discouragement is the last thing we want. Small improvements can be made by asking questions such as "Can you think of more words to describe that?", "What else could you add to show everyone what you mean?", "Do you have more of that picture in your head that we could put on the page?"
These questions are obviously about encouraging the child to write more, to create a bigger word picture. The picture will already be in their head - they just need help to put it into words. I like to describe poems as word pictures.

Suggestions for class poems
(These are starter lines, so every child uses the starter to get going - it's up to the teacher to decide if the repetition of the starter line will stay in the poem or become the title - some starter lines can become too dominant in the poem) :

If I could be any animal, I'd be... (the child should name the animal and say why, or use a description)
Example: If I could be any animal, I'd be a wombat, round and slow, sneaking out at night, burrowing in my hole.
Or: If I could be any animal, I'd be a cat because cats can balance on high fences, land on four paws and snooze in the sun.

I never told anyone ... (can be an imaginary secret or a real one!)
Example: I never told anyone that I swam the Pacific Ocean and wrestled sharks and took a shark fin home for dinner
Or: I never told anyone that my baby name was Bighead.

People think ... but really ... (again, can be imaginary or real)
Example: People think I'm just a kid but really I'm hungry vampire with long red teeth
Or: People think I love dogs but really I want a pet pigeon that will always fly home to me.

Thirteen Ways of Looking At ... (loosely based on the Wallace Stevens poem Thirteen Ways of Looking at a Blackbird - the number depends on how many students contribute). You can choose your city or town, your school, the ocean, the world - subjects for this are limitless. This is a good poem to follow earlier, simple class poems as it allows them to spread their writing wings a bit and write longer segments, up to six or eight lines if they want.

When you have compiled a class poem, read it out to them (as teacher, you can decide on the order of their lines to provide variety), and then print it up and put it on the wall.