I had a guy show me a trick and I couldn't figure it out for the life of me.

He had a piece of paper taken from a hotel lobby (where we were) and he asked for a shape, I said "Triangle" and he drew a triangle.

He folded it into quarters and I wrote my initials on both sides, I opened it up while he went into another room and wrote "Cletus" in the middle of the triangle.

When he came back, he took it and ripped it up and knew what I wrote. I don't know if I'm just dense or didn't look at the right thing at the right time but I missed it totally. Can anyone help?

(PS - another used "Molly is Stupid" and he knew that too)

Maybe he used the center tear on u. That your best bet.

Where can I find this knowledge?

it was a "center tear". Pick up any beginner's magic book and it should be there. edited it is even in the "Klutz book of Magic" You can get them at Border's I think. But if you are really looking into mentalism..."13 Steps of Mentalism" is a typical beginner guide and does have various techniques including the center tear. Please refrain from asking for people to "instruct you" how to do certain things.