As Scott mentioned, grayed-out fields are copied or computed from elsewhere on the sheet.
Classes are entered under the Configuration tab.
For Skill attributes, hover the mouse to the left of the Class Skill checkbox for the skill. You should see a gear icon appear. Click that and a details subwindow should open for the skill. In this subwindow you can change the attribute used for the skill, add notes, and other options, depending on the skill.
You may want to read more about the Official Pathfinder sheet in the wiki: https://wiki.roll20.net/Pathfinder_Official