Most types of telescopes come supplied with one or two eyepieces; you change the magnification by switching eyepieces with different focal lengths.
The objective's focal length (F or FL) is the key to determining the telescope's magnification ("power"). This is simply the objective's focal length divided by that of the eyepiece, which you'll find on its barrel. For example, if a telescope has a focal length of 500 mm and a 25-mm eyepiece, then the magnification is 500/25, or 20×. Most types of telescopes come supplied with one or two eyepieces; you change the magnification by switching eyepieces with different focal lengths.