Alphabet
Written Japanese combines three scripts. Hiragana and katakana are phonetic syllabaries - each character is exactly one sound - while kanji are meaning-based characters borrowed from Chinese. Learn the two kana sets first: together they cover every sound in the language and are the foundation for everything else.
The Japanese writing system
Hiragana (ひらがな) - the rounded syllabary of 46 basic sounds, used for native words, grammar particles, and word endings.
Katakana (カタカナ) - the same sounds in an angular form, used for loanwords, foreign names, and emphasis.
Kanji (漢字) - meaning-based characters borrowed from Chinese; you pick these up gradually through the JLPT courses.