As everyone else has said, more riding to build up more endurance. Also, as someone here has suggested make sure you have enough tire pressure. Of course, having the right seat height position is important. Finally, I would like to ask what kind of tires you have on your bicycle? It is common practice these days to sell so-called commuter bikes, with wide, knobby tires that almost look like off road mountain bike tires. There is no reason to use wide, knobby tires if you are riding primarily city streets. These wide knobby tires just slow you down and make you "tire" easy.