I've recently bought a reflecting telescope with a focal length of 650mm, which lens should I use if I want to see saturn and its rings?

If you are pointed at the right thing and in good focus 65X is enough magnification to recognize the disk of Jupiter and Saturn's ring. They will be small, but visible.
Do you have a finder scope? Point the telescope at the moon, because there will be no mistaking that for something else. Is your finder in alignment? If not, align it.
Saturn rises approximately 10 pm.
Jupiter is brighter than any star in the sky. Saturn is in Libra. The nearest first magnitude star is Antares, and Saturn is brighter. Try using your 25mm eyepiece to aim the telescope. That will give a wider field of view. It will give you 26X, which was about the magnification that showed Galileo their was something peculiar about Saturn.

Well, ..., the suspect reason why they show up as tiny dots is because your scope has a small aperture (I'm guessing 60mm). You need at least a 6" Aperture with a 1200mm focal length to begin to determine anything about ring structure. But even with a 6" scope the planets will be small. Astronomically speaking planets are very small objects.

Try an objective lens with 6 mm focal length. I don't need to know all the technical details of your telescope because I understand optics and I'm not nosy.