こんにちは is correct as it is actually a set phrase. It means 'as for this day'. Kon is this. Nichi is day. Ha is actually pronounced wa as it is the particle/topic marker. For eg. Watashi wa (わたしは) is the same principle. 'As for me'. But as I understand it, こんにちわ has also been used but isn't the one to use if you want to write properly.

I think you want to know the Japanese translation for 1. and 2. and the answer for 3., don't you? If not, please ignore my answers. 1.How to say "..... " in japanese? --> "..... " は日本語で何と or 何て言いますか。["……" はにほんごでなんと or なんていいますか。] 2.which one is more correct? --> どちらが正しいですか。[どちらがただしいですか。] 3.こんにちわorこんにちは? --> 「こんにちわ」と「こんにちは」のどちらが正しいですか。 The correct is こんにちは and it's pronounced こんにちわ. Some Japanese people use こんにちわ in their casual text messages between friends but it's incorrect. I hope this helps you.