If you use a good, comprehensive textbook for learning Japanese then you will be learning kanji gradually at the same time as you learn the language....which I believe is a more effective and practical way to learn them than attempting to learn them as a separate endeavor (at least at first).
The entire language can be written in all hiragana (or all katakana), and it isn't unusual to just use hiragana for writing words when you either don't know or can't recall the kanji.
The entire language can not be written in all kanji, as kana are needed for things like particles and the end portions of verbs/adjectives (showing the different tenses).
You can write ひらがな or ヒラガナ or 平仮名 for "hiragana" (although it would probably be considered odd to write it in katakana).
You can write カタカナ or かたかな or 片仮名 for "katakana" (probably odd in hiragana).
You can write かんじ or カンジ or 漢字 for "kanji" (writing it in kanji is the most clear, as there are other words with different meanings also pronounced "kanji" in Japanese: 感じ or 幹事 as just a couple of examples. One major advantage of writing with kanji is that it helps to avoid confusion, as Japanese has a lot of homonyms).