I was wondering why は is sometimes pronounced as wa, for example in こにちは (it means hello I think) it's pronounced as konichiwa and not konichiha, am I simply pronouncing it wrong or is there something else?
You are pronouncing it right. When it's a particle (like a preposition in English grammar), it is pronounced "wa" for historic reasons*. When it's part of a word, it's "ha". For instance, if you're saying something about your mother, 母は (はは は) will be "haha wa". If you're saying something about a tooth, 歯は (は は) will be "ha wa".
In your example, it's "wa" because it's actually a particle. As if you were beginning to say something about today (kon'nichi wa, 今日は), but then you don't say what, because it's just a standard greeting and it's understood already that the meaning is "good day".
は when used as a sentence particle to indicate the topic of the sentence is pronounced wa and when used in words, it is pronounced as ha
(but similar to the confusion of irregular verbs, konnichiwa こんにちは mening hello/ good afternoon and konnbanwa こんばんは meaning good evening is some kinda exception in the language I will never understand...)
は when used as a sentence particle to indicate the topic of the sentence is pronounced wa and when used in words, it is pronounced as ha
(but similar to the confusion of irregular verbs, konnichiwa こんにちは mening hello/ good afternoon and konnbanwa こんばんは meaning good evening is some kinda exception in the language I will never understand...)
Both こんにちは and こんばんは are sentence openings that got "frozen" into greetings over time. 今日は (今日/こんにち) literally means "as for today...", with は functioning as a particle (same for 今晩は). They come from the first part of longer, open-ended greetings like 今日は良い天気ですね or 今日はご機嫌いかがですか, where the rest was omitted because it was predictable and could be left unsaid.
It's similar to how "goodbye" is a contraction of the old English blessing, "God be with you", or how you use "morning" as shorthand for "good morning".
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