A converging lens refracts parallel rays so that they meet at the principal focus. The distance from the centre of the lens to the principal focus is the focal length, f.
The shape of the lens decides how strongly it bends light. A thicker lens with strongly curved surfaces bends the rays more, so they meet closer to the lens. It has a shorter focal length. A thin, flatter lens bends light less, so its focal length is longer.
The power of a lens tells you how strongly it bends light. It is measured in dioptres (D) and is given by P = 1/f, with f in metres. A lens with a short focal length has a high power and a lens with a long focal length has a low power. For example, a lens with f = 0.5 m has a power of 2 D.
So to make a lens more powerful, make it thicker and more curved.