Shimano classifies its 105 road bike group as "'pro-level' technology more accessible to part-time racers and fitness enthusiasts." Shimano 105 sits in the middle of the company's road bike component hierarchy, just below the more expensive Ultegra and the stuff Lance Armstrong used to win the Tour de France, Dura-Ace. Changing gears with Shimano 105 shifters is easier and more efficient than on the lower-end models, such as Tiagra and Sora.
Rear Derailleur Lever
Locate what Shimano refers to as "lever A" in its technical document for Shimano 105 shifters. Your rear derailleur lever assembly is located on the right side of your bike just above the "drops" of your handlebar. As Shimano shifters and brake levers are part of the same assembly, lever A is the lever on the shift/brake lever assembly that you press in order to brake. It is the lever farthest away from you when you are on your bike.
Click lever A to your left one time to move your chain one gear from a smaller to larger sprocket (e.g., from third to fourth gear). Two quick clicks will move the chain two gears (e.g., from third to fifth), while three quick clicks will move the chain three gears (e.g., from third to sixth).
Locate lever B, which is the smaller lever located right behind lever A. Click lever B to your left in order to shift from a larger to smaller sprocket on your rear cassette.
Front Derailleur Lever
Locate lever A on your front derailluer lever, which is on the left side of your bike.
Click lever A one time to your right in order to move from a smaller to larger front chainring. Shimano notes that, at times, this shift will stop just short of completion. In this case, give lever A a quick micro-click to complete the shift.
Locate lever B just behind lever A. Click lever B to the right to move from a larger to smaller front chainring.