Jump in!
Will people steal your game? Perhaps, but first they'd have to notice it out of the 798,830+ other projects on itch.io. Getting noticed is the part most of us struggle with.
Is itch.io a great place to start? Yes. The people behind the scenes work hard to make the place as safe and friendly as they can.
Will you get great feedback? Perhaps, you'll probably need to ask, and offer something in return - like show me yours and I'll show you mine. Sometimes people offer to play your game and give you feedback, but they're often inundated with requests. It also depends on whether your game is free or not.