In order to enter a prize you must be a photographer or a model (self-shot sets only), and the set must be unpublished on the Zivity website unless the prize is accepting old submissions. Click on the "Prize" tab at the top of the site and navigate the current prizes. Click on the one you want to enter.
There will be a button in the upper right hand corner that says "Submit Photo Set." Follow the regular process of uploading your set.