Soreness in the neck, shoulders, and butt should all be expected when you start riding and aren't used to the bike position and muscles that you are using. But---go to a good bike shop and have them check the fit of the bike you are riding. Proper fitting can make a huge difference. And yes, the saddle can make a difference too, but first make sure it is at the right height and correct fore/aft position and correct tilt.