If you use velocity to move your missile, you can use this code (maybe you will need to normalize it). writing from head.
rotation = -(float)Math.Atan2(velocity.Y, velocity.X);
so if your velocity is (1,0) meaning your misslile is going straight from left to right. Atan2(1,0)
will give you 1.57 radiants (90 degrees). to make this working, your texture of missile must be faced up. and this code will turn it to 90 degrees, to the right.