I'm glad you've been enjoying the game! The easiest way would be to start a new game by clicking "Start" on the main menu. All choices will be reset for a new game, so nothing you've done in prior playthroughs will affect your new game. The only data that is permanently unlocked during a playthrough are the gallery images, and the game will let you skip through any dialogue you've read in the past.
If you want to 100% delete all of your old saved data and reset the gallery to have all images locked again, you can do this:
1) Use File Explorer to locate AppData\Roaming\RenPy\FreshmenPhysicalEducation. The full path usually begins with something like C:\Users\[user name]\AppData\[etc.] but could be different based on your computer setup.
1b) By default the AppData folder is hidden. If you get into C:\Users\[user name] and don't see that folder, click "View" at the top of the window, and then in the Show/Hide section make sure "Hidden items" is checked.
2) Delete everything in the FreshmenPhysicalEducation folder.
3) Open the folder where the game is stored (the latest version is stored in a folder called "FreshmenPhysicalEducation-0.1.3-pc" unless you changed the name).
4) Open the "game" folder, then open the "saves" folder.
5) Delete everything in the "saves" folder.
That should do a complete reset, but let me know if you have any trouble!