If you ride in a very upright stance you put more weight on your bum and generally want a softer saddle. Riders who do longer distances prefer to shift more weight onto their arms by leaning forward more. They dont sit on the saddle but rather perch on it and find that using a harder saddle is more comfortable.
If you do anything more than 3 miles on a regular basis you may want to try the second method.

Cycling, like any new activity, stresses your body in new ways. It can take a short while to get used to this. Limit your rides to start with to about 20 mins and try and ride every day. After a week you will be in much better shape to try longer outings.
Make sure that bike is setup to be comnfortable with saddle and bars at the correct height for you. Also, dont push hard against the pedals, pick an easy gear and spin more rapidly.