The type of bicycle you are riding (and how you have decided to set it up) will determine how to operate your Shimano shifters. There are three main categories of Shimano shifters: road, mountain and hybrid bike style. It is not uncommon for a road cyclist to use mountain bike style shifters. Also, mountain and hybrid bike shifting systems are sometimes the same or very similar. Regardless of your particular circumstance, knowing how to manipulate your shifters makes riding more enjoyable.
Locate the shifter levers on your Shimano road bike. Modern road shifters are located on the handlebars just before the "drops" (where the bars form a sideways 'U'). Shift levers and brake levers are part of the same assembly. The right set of levers controls the rear gears. The left set of levers controls the front gears.
Click the lever farthest away from you on the right side to the left to move from a smaller to larger gear on your rear cassette. This lever is the same one you press when you wish to brake. Some Shimano road shifters allow you to move two or three gears at a time with two or three quick clicks to the left.
Click the lever behind the one you just used to the left in order to move from a larger to smaller gear on your rear set of gears.
Click the lever farthest away from you to the right on the left shifter assembly to move from a smaller to larger front chainring. Clicking the lever right behind the one you just used to the right will shift from a larger to smaller front chainring.
Locate your mountain bike shifters on your handlebars just past the grips. Most Shimano systems employ a "trigger" shift system. This set up is common on some hybrid bikes as well. As with road bike shifters, the left side controls the front gears and the right side controls the rear gears.
Push the right side bottom lever to shift the bicycle up to a higher gear on the rear cassette. Pull back on the top lever to shift the bicycle to a lower gear on the rear cassette.
Push the left side bottom lever to shift the bicycle down to a lower gear on the front chainring. Pull back on the top lever to shift the bicycle to a higher gear on the front chainring.
Look at the part of your handlebars where you place your hands when riding, known as the grips. At the end of each grip is a mechanism that can be turned in increments. This is the grip-shift.
Click the grip-shift as if you are operating a throttle on a motorcycle. Shift up to a more difficult gear by clicking the grip shifter away from you.
Shift down to an easier gear by click the grip shifter back towards you. This process works identically for the front and rear gears. Some bicycles will only have a rear set of gears, therefore only a right side shifter will exist.
From model to model, Shimano shifting systems offer slight variations in operation. The tutorials in this article provide instructions on the most common applications. Some lower-end Shimano road shifters require you to click a button on your brake hoods (the rubber piece at the top of your handlebar that covers your brake assembly), usually with your thumb, to move from a larger to smaller front chainring and to move from a larger to smaller rear gear. On the front of your bicycle, being in a larger chainring makes pedalling more difficult. On the rear, a larger chainring makes pedalling easier.