The goal of taking 10,000 steps in a day is a rough equivalent to the Surgeon General's recommendation to accumulate 30 minutes of activity most days of the week. It should be enough to reduce your risk for disease and help you lead a longer, healthier life.
First, invest in a pedometer.Keep it closed and attached to the front of your waist to the left or right of center. Wear it all day from the moment you wake up until going to bed, except when immersed in water. At night remove it, record the number of steps you've taken in the log, and note if you did any formal exercise (wear your pedometer then, too); for example, "20-minute treadmill walk."
Also note if anything caused more (museum tour) or fewer (all-day meeting) steps than usual in your day. Attach your pedometer to your shoe if you bicycle and the pedometer doesn't seem to count your pedaling.
If you want to know not just the number of steps you've taken, but the distance as well, you can calibrate a pedometer. The simplest way is to wear it while walking a known distance, such as once around a quarter-mile track, at your normal walking speed. Then multiply that number of steps by four, and you know your typical number of steps per mile.