First, Japanese is not so easy for language amateur to understand through Yahoo's Q & A. Please take a lot of time to study them.
Hiragana, Kanakana and Kanji are all Japanese characters.
Hiragana is sort of Japanese alphabet basically. Infant starts to know these first.
Katakana is similar as Hiragana which is actually corespondent to its Hiragana each one by one but usually for the words from foreign countries(borrowed words) or some emphasis or exaggeration.
For example, あ is Hiragana of Ah and ア is Katakana of same Ah.
There is no capital letter without some special exception in Japanese.
Most of all Japanese Kanji are from Chinese but the content may have been changed in some of them during the long history.
On speaking and listening, you don't need to consider which is which for these three characters but just need to construct Japanese grammar with Japanese vocabulary. Hiragana is essential for it.
On reading and writing, you must comprehend a lot of Kanji because individual Kanji is ideographic character and in addition there are many combination of Kanji so that you need to use proper Kanji for making exact text to reading and writing.
For example, happiness is しあわせ(siawase) or こうふく(kofuku) in Japanese Hiragana. It is not written in Katakana in a manner as commonsense.
しあわせ is also 幸せ or 幸 and こうふく is also 幸福.
As you see Hiragana and Kanji are combined sometimes ocassionally.
I am Tom is translated into Watashi wa Tom desu which is equal to
私はトムです。 or わたしはトムです。in Japanese.
私 Kanji
わたし Hiragana
は Hiragana
トム Katakana
です Hiragana
。 Japanese punctuation mark
http://en.wikipedia.org/wiki/%E3%81%B2%E3%82%89%E3%81%8C%E3%81%AA
http://en.wikipedia.org/wiki/Katakana
http://en.wikipedia.org/wiki/Kanji