To a beginner, cyclists seem to do some odd things. They wear tight clothing, they ride downhill in a tucked position, and they ride very closely together. Sometimes they ride bikes with odd looking bars and alien-like helmets.
The explanation for this odd behavior is aerodynamics. The main thing that slows cyclists down is drag. Drag is a backward force created by air friction. Cyclists (especially while racing) do all they can to minimize this drag.
Have you ever ridden on a windy day and noticed your baggy shorts and t-shirt flapping in the wind like a sail? Loose clothes create a large amount of aerodynamic drag and slow you down. The more form fitting the clothes are, the more aerodynamic you'll be, and the faster you'll go.
Drafting is also very important in competitive cycling. The reason cyclists ride so close together—the formation is actually called a peloton—is also because of aerodynamics: The riders in back can conserve energy by drafting the riders in front.
While downhilling, racers tend to get very low on the bike. This not only increases their speed, it lowers their center of gravity and makes cornering safer. Once they hit around 40 mph on a road bike, pedaling takes a back seat to aerodynamics. So, while going downhill, if you want to go faster you have get into a tuck position. If you aren't comfortable with this, try it first at lower speeds. Experiment with different positions to find the one that works best for you.
In time trials, a racing event where participants ride alone and are timed individually, racers get even more serious about aerodynamics. They wear longer, pointier helmets with fewer vents and ride with aero bars—those strange looking forward-pointing bars that let the rider bend down under the wind by resting their arms on the handlebars.
So, if you want to go faster, pay attention to wind resistance. As a beginner, you should practice riding in a group and riding in an aero position on the bike. Ease in at a comfortable pace for you. Advancing too quickly can be dangerous for you and those around you. And crashing makes you go slower no matter how aero you are.
Practice your aero position at a cycling race.