There are two fundamental concerns when creating or improvising. The first is having something to say. The next is having the guts to say it. Actually those are in reverse order, until you get over the guts part. The hardest thing for the novice improviser, generally, is to trust him or herself enough to jump in. You’ve got to learn to trust yourself, your instincts and your taste and stop thinking and worrying about what you just played. Easier said than done, right? This is a very hard thing for people to do, especially adults. It’s the spirit of play. Children know this. A child isn’t concerned whether you can see the dinosaur or the pirate in the bedroom. He’s not concerned when he uses a green crayon to color the sky or that Bo Bo the Frog talks with him. His imagination is vivid. He’s creating with great abandon. But the adult has excuses why he can’t do this or that, or how this is terrible or how he can’t possibly play like that or let himself go. He gets embarrassed and puts himself down before he tries. And then he criticizes others creations and listens to criticisms, or imagines them. The problem is he has very sophisticated tastes and has a hard time listening to himself play so poorly. He KNOWS what's good and what isn't. This is the hardest stumbling block.
It’s best to begin by taking small steps first. In a small space on your instrument, give yourself a few notes and just play with no rules. There are no mistakes. I used to play free or atonal music, which was PERFECT for starting out. Just get used to playing anything. Play. Just play and let yourself go. Refinement comes later. Anything you play is OK.
Then work on the next part: content. Start with scales perhaps and play around with those notes without any rules. Then maybe play that scale against chords that work with that scale and play around with that. Or play against rhythms. There’s a whole, huge body of information to ingest. Some of what becomes your style is what tools you use and how you use them.
But the big ogre here, as I see it, is THINKING. Thinking is like quicksand. You must learn to respond. You must intuit. You must KNOW when you play, even when you don’t. But THINKING is slow. Intuition yes. Thinking no. Yes there’s a lot of THOUGHT in playing jazz, or any improvisational music. In the beginning there's a LOT of thinking. Eventually there's almost none. We’ll get to this conundrum later.
The first rule in the art of improvisation is to trust yourself and to let go. Suspend your disbelief in yourself and trust that you have taste. It will come out, eventually if you let it. You have to decide it’s ok if you sound bad. It’s ok if you fail, for a little while. Playing is a process of learning and becoming acquainted with yourself.