I'm doing it at the moment

(Well, have been for about 3 years, 2 years properly).
My advice is to start really simply.
1) I learnt hiragana + katakana from a book called Japanese for Busy People and playing this flash game :
http://www.easyjapanese.org/kanaquiz.html ).
2) Get as much "media" as you can and immerse yourself in it - I listen to japanese music most days, I also got the "dummies" book and listened to the cd and read the book.
3) Get a japanese friend! I have a friend in japan now called Ayako, she is a sweety, and another girl in brighton called Yuumi, who is also a sweety and a complete babe. Hang out with them and chat! Learning a language is really cool and you will make lots of friends!!
4) Try lots of japanese stuff and ask what they are called!
5) Go on a course! I am on a part time course at Sussex University, great japanese guy teaching it, and having set structure and a group you can work with can be really motivating.
6) Set yourself some goals! What is it you actually want to achieve? What level of fluency? You want to be able to read a japanese newspaper? Its possible! Anything is possible...