control the curve by using the white direct selection tool to move the handles round. try moving them further away from the anchor point, following the same direction, and see how this changes the curve.
we only want a curve on the upper side of the anchor point, so we will first of all remove the handle on the lower side. you do this by simply picking up that lower handle, using the white direct selection arrow, and drag the handle back onto the anchor point. you should "feel" it snap away. drop it there, and you will now only have a handle (and thus a curve) on the upper side 