I'm with
@Stwert here: start by practicing. Then practice some more. Then when you feel like your times aren't improving, learn from guides or other speedrunners. Then continue practicing.
And to be honest: I think that "I won't be good enough" is more a problem than anything else. You already know how to get into speed runs: just start playing a game and finish it as fast as possible. Sure, it won't be as lightning fast as what you see on some youtube channels, but what you see there is the result of many, many hours of practice. What you should think about is "is this worth pursuing?".
As an example: when I started karate, the black belt karatekas could do all this amazing stuff. Seemingly incredibly complex moves. Sharp and hard moves. If I had just looked at them, or had said something like "I am thinking of going into karate, but I'm not sure how to get into it", then I wouldn't be where I am today. Instead, I looked at fellow white belters, trained, learned, had fun, and took every opportunity to improve, even if only a little bit each lesson. Heh...it's been about five years since, and I still do that (I'm currently one step away from a black belt, though I have to admit it's by far the largest step). I suggest you take the same approach: do it because you want it. That, combined with an idea of the goal you work towards, should give you all the motivation you'll need to improve.