Determine your solution's pH if your instructor of textbook doesn't give it to you outright. If you have a "pOH" value, simply subtract that number from 14 to get the pH. For example, if your solution has a pOH of 11.2, it will have a pH of 2.8. If you have a solution with an unknown pH, measure it using paper test strips or, if you have one, a pH meter.
Plug your pH value into your scientific calculator. Keep in mind that the pH represents that negative logarithm of hydrogen ion concentration---in other words, hit the "+/-" key to make your value negative.
Hit the "10^x" button to calculate the "inverse logarithm" of your pH---hit the "=" button a second time if your calculator doesn't immediately express this value in decimal form. For the example solution with a pH of 2.8, this value is .00158, or 1.58 x 10^(-3) M hydrogen ions, where "M" represents concentration in terms of "moles" of hydrogen per liter of solution.