----Hiragana は and わ ----
The sound *'wa'* is always written with the Hiragana わ such as in わかり ます (wakarimasu) for "I understand". However there is one EXCEPTION to this rule. The topic marker!
When *'wa'* is the topic marker - it is written は. When は appears somewhere else, it is read *'ha'*, as in はい (hai) for "yes" - BUT as a topic marker, it is always read as *'wa*'.
It is a bit tricky when you want to write the sound *'wa'* - because you have distinguish between whether you're using it as a topic marker (use は) or as something else (use わ).
On the other hand, you never have to wonder how to write the sound *'ha'* - it's always written は.
:idea: こんにちは is written with a は because the は is actually a topic marker - こんにち literally means "THIS-DAY"!!
So, literally, when you say こんにちは you're saying "THIS DAY IS..." :P