To complete the square, first you must make sure that the coefficient of the x^2 term is 1. Here you have x^2 - 2x - 37 = 0. Since the x^2 term is simply x^2, you do have a coefficient of 1 for the x^2 term.
To find the number you need to add to complete the square, take the coefficient of the x term. Divide it by 2. Then square it.
The x term is -2x. The coefficient of the x term is -2.
