You could use the free app Duolingo, which recently added Japanese. You should learn at least the hiragana to help you read and write. Textfugu has some charts for hiragana and katakana. (Hiragana and katakana are sound-based character sets analogous to cursive and printing, respectively. Hiragana is generally used for Japanese words, while katakana is generally used for things like non-Japanese words. Once you've learned to read and write all the sounds, you can express yourself phonetically in writing. The kanji are more complicated and are based on meanings, rather than sounds.) Jisho.org ("Jisho" means dictionary) is a great resource for translating words, but it uses Japanese characters, rather than Romaji, so it will be difficult to use if you don't know Japanese characters. You can use RomajiDesu if you're used to the English alphabet.

Also, a word to the wise, textbook Japanese (with formal grammar and everything) is different from more casual speech that might be heard in Japan or in anime. Figuring out the new grammar might take some getting used to, so it might help to just try to focus on understanding the words being used at first.