If you are trying to learn......?

We use kanji, hiragana and katakana exclusively. Romaji is just used for foreigners and in logo of company or something.

I recommend to memorize hiragana and katakana. They only have about 100 characters and are phonetic symbols so you can get to know how to say Japanese.

But kanji has over tens of thousands. You have to memorize at least 2,000 kanjis and lots of kanji compound words to read Japanese. Try to memorize kanjis after you learn basic Japanese.

All four of those forms are normally used in everyday japanese. Hiragana is normally used to modify Kanji (chinese characters) so that it can be more cohesive to the Japanese speaker (its like the word big, when modified it becomes bigger or biggest). Kanji has two types of reading; the Japanese way and the OLD Chinese way, both are used, thats why you will see a fifth form of writing, which is a Kanji with a smaller hiragana on the side, so that you pronounce the word correctly. Katakana is used for foreign loan words and sounds(this is a newer trend...America and other words that were introduced earlier were actually given Kanji characters). Romanji are western letters (like the ones you are reading now). Hiragana and Katakana are the first two types of characters that you need to learn, to fully understand and undertake the Japanese language.