Well は is always ha and only becomes 'wa' when it is used as a particle eg. わたし は。 it also becomes a wa sound in certain words like 'konichiwa' 'こんにちは‘ as konnichiwa was originally 'as for this day'. Its a set phrase and the は is a particle in this word.

I agree with Jessica. When it is a particle which is used after the subject, it's written は and it's read WA. When it is a part of one word, it's written and read HA. Ex.) HA: はじめまして(nice to meet you)、はし(chopsticks / bridge)、は(leaf)、はな(flower / nose) これはさくらのはなです。--> kore wa sakura no hana desu. わたしははしをつかいます。--> watashi wa hashi wo tsukai masu. *は after これ and わたし is a particle, so it's pronounced WA. I hope this was helpful.