Whatever method you use, just remember that you are learning to read a new language. It will take you about the same amount of effort to read music fluently as it would to read Japanese fluently (assuming English is your first language, and you have no prior experience with music notation). Ear training is important too, so that you can recognize the sound of each note while you're reading it, even without an instrument in your hand.
Dedicate some time each day to the task, and give yourself a little test every couple of weeks. If you do this for a year, you'll have the basics. If you do this for two years, you'll be pretty darn good! In three years, you'll be pretty darn fluent and should be able to sight read/play all but the most difficult pieces on the spot. Good luck!