I'm trying to learn how to read and write in Japanese
 

So right now I'm trying to learn the Japanese language, I've been studying the speaking part but now I want to read and write Japanese in Japanese internet forums because I think that would really help me understand the Japanese by talking through internet forums since I don't know any Japanese people. I understand that theres four forms of Japanese writing called Kanji, Hiragana, Katakana and Roman character which is the English alphabet. If I studied the Hiragana writing and fully understood it, would I be able to understand the words through Japanese forums or do I have to learn the other forums of writing as well?

And no one on internet forums writes only in hiragana.