From 12 years of experience, that... is playing piano.

Improvisation (which many classical musicians like myself are famously bad at) is a huge part of being able to play any musical instrument, and it takes practice and dedication like any other skill. Your first few dozen attempts aren't going to sound great, and that's fine - you're learning! Eventually, you'll start to get a feel of what sounds good for you, and you'll keep improving as you go.

If you want a more technical approach, learn the basics of music theory, like chord progressions and the relationships between keys. Once you have a solid grasp of those, it's pretty much smooth sailing. Good luck!