Japanese uses a mix of kanji and kana (hiragana and katakana)
as far as usage goes, any, if not all, can be used in a sentance. so there is no "just learn ____ and you'll be OK because it's the only thing used".
to
generalize:
kanji and hiragana are the most common form of writing in sentances
katakana represent foreign words, sounds or emphasis on specific words
kanji usually denote names of things, titles, (nouns), adjectives and verbs
don't get me wrong, there is no rule as to how many kanji/hiragana/katakana are required in a sentance.
this may help a bit:
Japanese language - Wikipedia, the free encyclopedia