Try what the previous post said, and make sure that the cable is seeded in the shifter correctly. I'm sure Sheldon Brown has an article somewhere on how to do this. But if you still don't have any luck, you might have a broken shifter. I had a similar thing happen to a shimano 105 front shifter on my bike a little while back and have seen it at my bike shop on shimanos a couple times. Before making any decisions on this though, take the bike back to the shop you bought it and have them look at it. If indeed the shifter is broken, shimano has a one year warranty on parts.