To boost your post firstly you must create one, the guide to do this can be found here.
While creating your post you will see 'Hottest post boost'
This is where you have the ability to push the post further up the hottest wall. To do this input a multiple of ten, 10-20 should do the trick!
Not only can boosts be used to push posts up the community wall, they can be used to suppress them. Negative boosts can be entered to push the post down the hottest wall.