pH is the negative of the log of the hydronium concentration. Take the log(1.0E-3). That gives you -3 because the log of 1 is zero and the log of E-3 or 10^(-3) is -3. Now multiply by -1. The pH is 3 and this is an acid solution. You can put the whole number into your calculator to find the log. Then push change sign. Be sure you are using log, not ln.