You can find the manual here:
https://ismacs.net/singer_sewing_mac...e-manuals.html
Scroll down to find the 115 manual. It has a diagram with the points that require oiling. Be sure to use sewing machine oil, not household or 3in1 oil. Turn the hand wheel by hand a few times with a drop added at each oiling point. It helps work in the oil, and allows you to feel the difference as the oil works in.
Also check to be sure the bobbin winder tire is not engaged.
The 115 is incredibly smooth. When you practice treadling, I recommend doing so without it being threaded. Get used to stopping and restarting, ensuring the hand wheel turns toward you. If the hand wheel runs backward/clockwise, and it is threaded up, you will get a thread jam in the bobbin area in very short order- ask me how I know!