The first things we learned in our Japanese class were simple phrases and whatnot on how to introduce yourself. I think for a few days you should just learn how to pronounce some words using Romanji and then work your way up to Hiragana and Katakana.
Learning simple phrases such as: Ohayoo gozaimasu! (good morning [polite]) and Watashi wa America-jin desu. (I am American.) Is where you want to start. Learn some nouns for words such as cat (neko), dog (inu), etc. etc.
Once you feel you got a few words down, and can say some short phrases, start learning Hiragana. Now practice those words or phrases by writing them out in Hiragana (even ones that are suppose to be written in Katakana at first). After you feel you got somewhat a handle on Hiragana (you don't have to remember them all, but 80% would be good, the rest will be better remembered when you start using them) start learning Katakana. Learn how to write your name in Japanese by using Katakana. Remember Katakana is only used for foreign words or words you want to emphasize, well...there is special cases but don't worry about that now.
Get your Hiragana down to where you feel semi-comfortable writing in it. Practice reading simple sentences, if you can find a children's book in Japanese that would be great.
Once you feel like you are at a level that you can at least introduce yourself in Japanese, not a detailed introduction, but maybe your name and where you are from would be sufficient along with some basic greetings and whatnot and can write pretty well in Hiragana and Katakana you should start learning some basic Kanji. Again with Kanji just try learning some easy ones such as big 大, what 何, food 食べもの, etc. If you want to be serious about it, try to memorize 2 kanji a day. Remember you need to know what it symbolizes (word) and also the sounds it will make when you read it. It is daunting but you will eventually want to know 2000 Kanji or as close to it as you can get.
Good Luck to you, the younger you are the easier it is to learn a language, and if you plan on going to University more than likely they will offer a Japanese Language class (日本語のｸﾗｽ) and to have some previous knowledge will be very beneficial.
Picking up a dictionary along with a Kanji Dictionary will be extremely helpful. Remember when you learn your Kanji to follow stroke order, it is important! Here is a online dictionary that seems quite accurate. Type WWJDIC into a google search engine. (I would link the site but I don't have enough posts to do so)
Hope this helps, and have fun!
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