Keep your chain oiled and make sure your tires have enough pressure.
Your bike might be out of adjustment, too. Make sure your chain is not too tight or too loose. A tight chain is harder to pedal, and a loose chain falls off.
Pretty general question with no details. So a general answer is tires. Thinner, lighter, smooth, hi pressure tires make more difference than anything else you can do to a bike to increase speed. And keep them pumped up.
Also, make sure your wheels are in good shape: are they straight and don't rub the brakes? Are your wheel bearings smooth, lubed and adjusted?
better tires, pump up your tires, keep you chain oiled, get lighter parts, work harder ect.... if you are going to get new parts, start with important parts like the chain, derailers, pedals ect...