Knowledge resembles a tree.
The core of a subject – the trunk – makes up the foundation of that discipline.
If you're trying to learn cooking, for example, the trunk includes things like basic knife skills, how to measure ingredients, and how to use various heat sources.
After that come the branches. These aren't quite as essential as the trunk, but they're important to learn on your way to mastery.
Things like mise en place, choosing the right kind of pan for each dish, and understanding the chemical reactions that occur during the baking process are branches of the culinary arts.
And lastly we have the leaves. This is the specific knowledge that you'll only use rarely, but is what distinguishes experts from the merely experienced.
In our cooking example, leaf knowledge might include how to extract every ounce of meat from a lobster, techniques for perfect poached eggs, and how the coarseness of different salts affects your meal.
Part of what makes learning something new intimidating is the number of leaves you'll inevitably encounter early on. Without the trunk and branches to mentally hang them off of, you'll struggle to make sense of them.
So start by identifying what the trunk of your topic is. Once you've reached the top off it, figure out the branches that extend out of it. This gives you a sense of what you don't know.
And knowing what you don't know yet is half the battle towards gaining proficiency.
See also: Latticework of Ideas