What Do I Need To Know To Compose Music (Piano)?

Hi! First post. I recently discovered that I want to learn how to compose piano music, mainly romantic and classical. I've played musical instruments for a long time (I'm 16) so I know at least the basics. I found a great website (Noteflight) that allows me to try composing things easily but I've come across some problems. I feel a little lost when it comes to getting the harmony to match the melody and chords sounding good next to other chords...I'm probably not explaining this well. I just want to know what I need to know to compose complex piano music so I can learn it. Thanks!

Start doing some reading, my friend.  It's a long journey!  Try googling "chord progressions" and see if that answers some questions.  When you are just starting out, it's hard to know what questions to ask, since there is so much to learn.  Start reading and let it all just wash over you for a while.  When you have specific questions, come here and ask for recommendations for more topics to research.  (:

Thank you very much! I'll get started.

Play as much music as possible and learn from it. That's the best and most complete way of going about it. 

If you want a more formal and structural guide for classical/romantic music, look for Arnold Schoenberg's "Fundamentals of music composition". Also, look for a good harmony book and maybe a counterpoint guide.^^

Both great suggestions, thanks!

Searching this site for anything music related to composition is the first step. There have been many posts like yours.

Also, 

WELCOME TO THIS COMMUNITY OF AWESOME COMPOSERS

Yeah...I figured there were others like this but I wasn't really sure what to search. Plus I had some specifics, but anyways thank you!

I've been watching a youtube series on music theory and I've already learned a lot. Still not really ready to compose anything but I'm getting there.

Get out now before you've wasted your life.

I think if I could waste my life doing anything it would be this, haha.