I'v made a very simple telescope !?

The power of a telescope is the focal length of the objective lens divided by the focal length of the eyepiece. In your case its 250/50 giving you a magnifying power of 5. The longer the focal length of the objective the more magnifying power you will have. Conversely, the shorter the f/l of the eyepiece etc. etc. Keep in mind that as you increase the magnifying power you will also have to increase the diameter of the objective lens since the image tends to darken as you look at a narrower slice of... whatever it is you're looking at. What you have is called a refracting telescope and they have limitations because of the way they work. Lenses distort the image because of the way light behaves as it pes through them.