Not long ago, shifters on racing bicycles were located on the downtube of the frame. Shifting gears therefore meant removing a hand from the handlebar and blindly reaching down to change gears. The practice was not altogether efficient or safe. More recently, shifters have been integrated with the brake levers, conveniently located on the handlebar. Shimano Flight Deck shifters are an example of this design. Mastering these shifters takes some getting used to but leads to increased efficiency and ease of shifting.
Place your hand on your left side shifter. Rest the space between your thumb and forefinger comfortably on the brake hood.
Push the brake lever into the bike to shift from the small chain ring into the large chain ring. This will create a larger gear ratio, making pedalling harder.
Push the small lever behind the brake lever into the bike. The will cause the chain to shift from the large chain ring back to the small chain ring. This is a smaller gear ratio, for easier pedalling.
Place your hand on your right side shifter. Again, the space between your thumb and forefinger will rest upon the brake hood.
Push the brake lever into the bike to shift the chain to a smaller gear ratio. When it comes to rear gears, do not be confused by the size of the cog. Contrary to most assumptions, bigger cogs create smaller ratios and smaller cogs, larger ratios.
Push the smaller lever behind the brake lever to shift to a greater ratio. Again, you will be shifting to smaller cog, but a larger ratio. Peddling will be more difficult, but your bicycle will travel farther with each pedal revolution.
Practice your shifting in a car park before venturing out with your riding buddies. It's best not to be distracted by the how-to practice of shifting while riding in the company of others.