Noteflight is good if you already know music theory. If you don't, it's useless until you do.

@will19 if you don't know music theory (how to read music and etc.) How to Read SheetMusic (readsheetmusic.info/basics_of_counting_rhythms.shtml) is an excellent way to start. Just take it slowly. The pages are deceptively short. Each page should take you quite some time to study and memorize. Remember, you're learning to read Egyptian hieroglyphics here.

@Yaowen I hope you have a dream like that one day. Waking up after singing along to the radio is just as energizing as singing along with the radio in real life. Especially if it's one of your all-time favorites. Mind you, I usually wake up and realize that not only have I never heard that song before, It's not even close to anything I've ever heard. When you write a song in your dreams and remember enough of it to write it down when you wake up, it really holds a special place in your heart. Oh, and check out that site I told will19 about. I'd love to know what you think about it.