Start with a seemingly simple question: is what I believe true?
This question actually is a lot harder to answer than it might seem at first glance, because nearly every answer you come up with will lead you to further questions: why do I think it is true? Are those good reasons to think it is true? What makes a reason a good reason? What makes it a bad reason? Why have I accepted bad reasons in the past? Am I still doing so now? How do I get better at ignoring bad reasons and sticking to good reasons in the future? The list of questions never ends.
And it shouldn’t. Question everything. When you find an answer, question it again. Don’t try to find ways to confirm that you are right, try to find ways to prove yourself wrong. Hold every belief tentatively, with the understanding that one day you may find out it is incorrect. Proportion your confidence in a belief with how strong the reasoning you have for holding it is, and make peace with the fact that your confidence should never hit the 100% mark.
Question today. Question tomorrow. Question always. This takes time, and effort. It will force you to face some harsh realities about yourself and your world-view. It won’t always be fun, and you’ll end up feeling like you’re an idiot more often than you’ll want to admit.
But it is worth the effort. Being rational isn’t a thing that you are, it’s a thing that you do, and the more you take the effort to do it the better you’ll get at it.
As for outside help, there are a lot of places to turn. Reading up on philosophy of epistemology isn’t a bad start, but it can be dry stuff. Playing games or watching shows/films that are intended to make you think and question rather than feeding you a “correct” answer can be extremely beneficial - in all seriousness, I credit watching Star Trek as a kid as being what pushed me onto the path of a skeptic and taught me to question what I’m told rather than simply accept it, to keep my mind open to unknown possibilities. I credit chess with helping me learn to analyze, to look beyond what is readily apparent, and to accept that I’m going to fail in this at times - sometimes embarrassingly so.
But being rational isn’t about being right, it’s about learning to be less wrong, and getting better at being less wrong over time - and there is incredible freedom of thought and act to be found in that. It’s a journey that is ultimately one you have to undertake on your own, for your own reasons and in your own way, and it is a journey without end.
But, to quote another favorite piece of fiction of mine: Journey before Destination. And that you’re already questioning means your journey has begun.