Well, having a sore bottom when just starting cycling is to be EXPECTED. It is normal. Keep up the riding, and you muscles will get toned and you can ride for hours without undue discomfort*. Now, the pain could be something more serious like a poor bike fit. Bike fit can usually be diagnosed and resolved by the Local Bike Shop, LBS, people. The other thing may be a poor fitting saddle. At the base of your pelvis are two little knobs of bone. These are called the Ischeal Processes. These processes are about 2.5" apart. Your weight on the saddle should be set on these two bones, that way your tender parts are not squished causing "discomfort". If the saddle is too narrow, the saddle will ride up between the processes causing discomfort. If the saddle is too wide, some weight will be borne by other parts of the body than the processes causing discomfort. One of our LBS has a little device for measuring the location of the ischeal processes and can set one up in the proper saddle. You might have one near you with the same gear.
* in the medical community "discomfort* means excruciating pain.