Hmmmm.....

Well. Like the guy before posted, you may just need practice. However, assuming your biking skills are intact, perhaps the headset or handlebars are loose.

You may want to go back to your local bike shop where you bought the bike, and have the guy there take a look, just to make sure evrything is up to snuff. Show the mechanic what is happening, and have him watch you ride it with the problem ocurring. Maybe he can even give you a few pointers.