The Japanese language has 3 alphabets: Hiragana, Katakana, and Kanji. Why 3 alphabets? They each have different uses.
ひらがな (hiragana) - this is the standard alphabet. These 46 characters are used for native words (48 if you count the obsolete WI and WE characters). Think JAPANESE words. Hiragana is also used to write everything that kanji can't cover, such as particles.
カタカナ (katakana) - this is their alphabet for foreign words, or all other words that aren't native words (46 characters also). For example, if Japan borrows a word from another language, such as ORANGE, it is spelled using katakana (オレンジ = ORENJI). In many cases, reading a word in katakana that is borrowed from another language such as English, you might be able to figure out what the word is since it is written to sound like it's supposed to from that language( ORANGE = ORENJI, CLASS = KURASU). Some words will combine both katakana and hiragana (i.e. サボる = skip class). Other uses for katakana include emphasizing a word (similar to writing all caps in English), or if you're a manga reader, it's used for sound effects.
漢字 (kanji) - Chinese characters. A single kanji character may have multiple readings, may sound similar to its original Chinese pronunciation, but may also have its own Japanese pronunciation. Kanji is used for JAPANESE words. That means Kanji is used in place of some Hiragana. For example, the word うみ (umi = sea), but in kanji it is this character: 海. Okurigana is when kanji is mixed with hiragana, such as 楽しい (tanoshii = fun). Furigana is when the hiragana readings are placed above the kanji so those unable to read the kanji will be able to.
It doesn't take long to fully memorize hiragana and katakana, but there are thousands of Kanji and only a few thousand are commonly used in Japan. It's easy to forget the kanji you've learned because there are a lot... but you're an eternal student, and you can do it. がんばりましょう！