OP - I wish I started learning when I was 14. Take advantage of all this time! You will thank yourself later.
I have been studying over the last couple years so I will offer you my advice from my recent experiences.
Don't bother with Rosetta Stone. The speakers are not native, and I didn't find the program very helpful. You may find it helpful, but I wasn't impressed with it.
Someone recommended
www.japanesepod101.com. I love JPod101. They are enjoyable to listen to, the speakers are native, and best of all the podcasts are free. They offer subscriptions to access all the accompanying content, so you may want to look into that to further solidify what you learn in the lessons. There are a range of audio lessons available from Newbie to Beginner to Intermmediate. They are working on an Advanced series soon, which will be good. Their forum is free and the community is very supportive.
Learn the hiragana and katakana. It doesn't matter which you learn first. Just pick one and go with it. I learned hiragana first and had no trouble with katakana afterwards. The method I used at first was pure drilling. I learned one column at a time, and continued to add a new column and review old columns until I could write the tables from memory. Then, start reading. Read anything, even if you can't understand it. This will solidify the kana in your mind.
Once you have this base, now it's time to venture into the kanji and grammar. Check out these sites. I've found them very useful. There are lots of sites out there to help you with everything.
JLPT Renshuu (kanji and vocab based on the JLPT's)
www.guidetojapanese.com (excellent grammar resource)
Jim Breen's WWWJDIC (great online dictionary)
Japanese Verbs (unfortunately only in romaji but covers a lot of verb structures)
Charles Kelly's Japanese Study Materials
MLC Language Center (check out the Free Materials page)
Hope that'll help you get going. Remember it takes time and effort. You will definitely get frustrated at times, but always look back and see how far you've come, as that will motivate you to continue on. You'll be surprised to find that Japanese is really quite easy (e.g. the verbs are very simple, and there are only 3 [commonly used] irregular verbs!).
Try reading as much as possible. Whenever you see a kanji you don't know, look it up. Actively looking up words increases the chances that you will remember it next time.
I can also recommend the Genki series. There is also Minna no Nihongo, which is relatively the same but covers some things that Genki does not. Look into those for textbooks. If you get Japanese for Busy people, make sure to get the Kana version.
Also, look into getting a kanji dictionary (such as Kodansha's Kanji Learner's Dictionary). Also, Kodansha's Furigana dictionary is quite useful as well.
Well, I hope I've helped in some way. Good luck and feel free to PM with any questions if you have any.