Here is the simplest way to fold an equilateral triangle from a piece of paper. This triangle will be as tall as the paper is wide. However, this won't work for a square sheet of paper because the side will be too long to fit on the page.
Steps
Fold the paper in half and open it again.
Fold the bottom right hand corner to the crease--making sure that the fold goes through the bottom left-hand corner of the paper.
Fold the top edge of the paper down along the edge of the paper that you folded up.
Open the paper to see the equilateral triangle formed by the creases.