How do I graph f(x)= -cos(x-pi)?

Just exactly as you graph any other function: List a bunch of values for 'x', find the value of the function for each one, and start graphing the points, until you have enough points to see the shape of the function, and can draw more of it without more points. When you do that, you'll discover that -cos (x-pi) is equal to cos (x).