As someone who just started last week, know this: Your ego will be as bruised as your body. Just last night I got swept, passed, mounted, and submitted by a guy 2 inches shorter and 100 pounds lighter than me. He has 4 months experience.
By the same token, I was able to pass and submit someone closer to my size (I only had a 60 pound advantage), and it felt great.
You will not be good, regardless of how much MMA you've watched. Just because you know what a triangle is, doesn't mean you know how to transition from closed guard to open guard control, into a triangle and properly execute it. I remember my first night we were learning triangles and I could not tap my partner no matter what I did. Turns out I didn't know shit about proper leg positioning or how to place his arms to tighten it.
Above all else, just be willing to learn