Start by looking up an exploit that has details published, and recreate it. Do that a few times, and you will begin to understand how it works without the frustration of not finding anything. As you get better you'll both get better at finding things and at dealing with the long periods of not feeling like you're making progress. I suspect there's sites that help you walk through this, and I know I've seen ones that walk you through prepared exploits.
You really just have to start putting in legwork consistently.
You really just have to start putting in legwork consistently.