Undecided questions
What's the best way to study hangul? and what's the easiest way to memorize katakana and hiranga ?

Just like Chinese. Chinese and Hangul are alike,they are both made up of different written-parts. Learn how the words are made up of

hi Mira..
i learn katakana too.. it needs less than a week for me to memorize it. i learn it by writing my diary in katakana. always. and it helps me a lot.
i also use java application on my mobile to take a quiz on my katakana+hiragana+kanji skill. or maybe if i forget how to write, i just check it on my mobile. so i can do it anytime, anywhere, without having to bring books of 'how to write kanji, haragana, and katakana' ^^
ok,
Good Luck!

James Heisig. He is the author of the only books you will ever need for hiragana, katakana, and eventually the kanji. You can only fool yourself for so long in believing you'll get along just fine without the kanji. Look for Jamese Heisig at your bookstore: Remembering the Hiragana; remembering the Katakana; Remembering the Kanji.