Yeah, as mentioned before theory is ESSENTIAL. Even getting the basics down is great. Learn a few scales, and chords that go with them, and that's an awesome start.
Also, never underestimate classical piano, it is a fantastic way to learn a huge mix of techniques and styles. As long as you know how to read music, classical is the way to go (for starters).
If you want to start improvising your own stuff, playing a basic chord progression in the left hand with a melody (using notes from the scale the chords are based on... see, theory's important) in the right can sound surprisingly good.
Most of all, don't get frustrated if things don't sound exactly how you want them to. I've been playing piano for 14 years (although some of that I was really young and not learning a lot) and my improvisations get exponentially better every year. Piano takes a TON of practice and hard work to get really good at, so just keep it up, and don't be worried if it feels like you're moving slowly, it'll come.
Good luck, keep with it, it's totally worth it! Great instrument.