Excerpted from Wikipedia Space Shuttle article:
. . To achieve low orbit requires much more horizontal than vertical acceleration. This is not visually obvious, since the vehicle rises vertically and is out of sight for most of the horizontal acceleration. The near circular orbital velocity at the 380 kilometers (236 mi) altitude of the International Space Station is 7.68 kilometers per second 27,650 km/h (17,180 mph), roughly equivalent to Mach 23 at sea level. As the International Space Station orbits at an inclination of 51.6 degrees, the Shuttle has to set its inclination to the same value to rendezvous with the station. . .
Excerpted from NASA HST SpaceFlight Shuttle:
. . .To travel that fast, it must reach an altitude above most of Earth's atmosphere so that friction with the air will not slow it down or overheat it. The journey starts relatively slowly: at liftoff, the shuttle weighs more than 2.04 million kilograms (4.5 million pounds) and it takes eight seconds for the engines and boosters to accelerate the ship to 161 kilometers per hour (100 mph.) But by the time the first minute has passed, the shuttle is traveling more than 1,609 kilometers per hour (1,000 mph) and it has already consumed more than one and a half million pounds of fuel. . .
The Shuttle in near-zero gravity, there exists extremely little friction, and as Newtons Law states, for every action there is an equal or opposite reaction. As the SS reaches such high velocity in takeoff, that speed accelerates it into space. When in space, the Shuttle naturally takes on an object that with the force of gravity pulling it constantly in earth orbit, will then output itself to almost 18,000 miles per hour. I'm not sure why only, 18,000 mph, but it probably has to do with ratios, weights, and earth versus zero-gravity equivalents. Math and science. Hope this helps. Read up on it real quick at the following websites. The 2nd to last one explains a lot.
By the way, if you're interested in viewing the Space Shuttle and/or the International Space Station pass over your city, use the very last link I provided. It's a pretty cool experience knowing that someone is over 300 miles above the earth looking down at you traveling 17, 500 mph.