Does 'wa' change into 'ha' when writing in japanese?

I often made the mistake of typing 'konnichiwa' - こんにちわ instead of 'konnichiha' - こんにちは。 My japanese friend corrected me by saying that the 'wa' at the end becomes a 'ha' sound - こんにちは。

Is this a rule in the japanese language that a 'wa' sound is written as 'ha'? If so, what about "わたし" ？

"wa" is written わ except in the single case of the topic particle, when it is written は. This is one of a very few cases (in modern Japanese) where there is not a one-to-one correspondence between kana and sounds. Another is the particle "e" which is written へ (normally "he").