You write ha when using it as a particle. And you use ha when saying こんいちは or こんばんは. https://www.youtube.com/watch?v=7yUXHx2DVsg this video explains it a lot better

You can watch my answer to another person on: http://www.italki.com/question/301048

Might be terribly wrong on this but you do only pronounce は as "wa" when は is used as a particle. The pronunciation as such occurs to make the speech more fluent. As in your example, try comparing pronouncing it as "ha" instead of "wa" and you should note that it sounds more stuttering and less fluent.