こんにちは　(ha)　is correct, but a lot of people use こんにちわ (wa) with friends or informally :D

It is written kon ni chi wa こんにちは As for your question, the "ha" is being used as a particle and is therefore pronounced as "wa"

Thank yoy

Be careful when you type in hiragana - you have to type ko-n-n(hit the n-key twice), -ni-chi-ha and you will get こんにちは If you don't hit the 'n' key twice you will get こんいちは　which is incorrect - and if you read it that way you won't sound the double 'n' sound when you say it. My Japanese friends are bemused that so many English speakers can't get this simple word right and I am convinced that it all stems from a problem with the way we enter it on the keyboard.