Just a correction> It's こんにちは As for the question.. You need to use は and read it as WA because it is working as a particle (topic maker). You never use わ as a particle. ex わたしはみしまです。(Watashi WA Mishima desu) - I'm Mishima. So if は is alone and working as a particle, you will read as WA. If は is a part of a word, then you read it normally as HA. But there are two exceptions as far as I know> こんにちは (konnichiwa) こんばんは (konbanwa) - Good evening Here you will read the last HA as WA, because they used to be particles a long time ago, but not anymore. Still read as WA though.

It is an exception. Usually, はsounds like "ha" (for example, はじめまして - hajimemashite). However, as a subject particle は is "wa" as part of a normal word わ or ワ = wa, for example in わさび (wasabi)