Taxiing is the aviation term for manoeuvring a plane on the ground. In real life, taxiing is difficult and takes work. In "Microsoft Flight Simulator X (FSX)," taxiing is done by simply pressing the acceleration and turning keys. Although the game is not nearly as strict, you can still add some finesse to your taxiing skills and follow the not-so-strict taxiiway rules.
Tap the "9" key (or the corresponding button on your controller) to gradually increase acceleration to 25 on the airspeed indicator and begin taxiing. Depending on the size and power of the plane you're using, you may need more or less acceleration to get moving at an acceptable speed.
Tap the "3" key to reduce throttle, which will slow you down. In FSX, you can generally taxi up to 35 on the airspeed indicator and still maintain control over the aeroplane (and not take off accidentally).
Maintain a controllable taxi speed while on trying to stay on the yellow centerline on the taxiway. Although you use the ailerons to turn the plane in the air, you must use the rudder controls to turn while taxiing. Press the "0" and "Enter" keys on the number pad to turn left and right respectively. All planes handle differently, but as you get use to taxiing with a plane, you will know how much acceleration and rudder is needed to taxi efficiently.
Taxiing is significantly easier by using a joystick or an actual flight controller (such as a yoke) instead of the keyboard.