Finding initial reaction rate of iodine clock reaction?

Hi, I have completed an experiment of the iodine clock reaction using hydrochloric acid, potium iodide, sodium thiosulfate and hydrogen peroxide. I have gathered data about each chemical, and I have preformed three experiments, in which I change the concentration of each the peroxide, acid and potium iodide. I timed the time it took for the solution to go blue, but now I have a problem. How do I calculate the initial rate of reaction in order to then calculate the order of reaction? Pleeease help!