I would like to learn how to finger pick a guitar?

I would like to learn finger picking on a guitar...but I have seen them on youtube i just don't see how they do it. How do they know how to put their fingers on the fretboard so easily while they are finger picking. I want to learn, but I am so confused how they play...and i haven't gotten a guitar yet.. I saw videos how they strum too. That part is clear, because they strum and they know where to put their fingers of the fretboard but how can you do that when finger picking when your fingers move so fast..and swiftly.. sorry if you don't know what i am trying to say o.o just bad at explaining. Thank you for helping me.