Magnification is a result of the eye piece and Barlow adapters you're using against the focal length of the tube. The main thing to know is the limits of your telescope's focal length. To put it simply, the more light collection (larger the diameter of the tube; also known as aperture), the higher magnification you can resolve, up to where atmospheric conditions limit ground-based optics (e.g. 300x).
Simple math for it is:
Focal length mm (milimeters) = Magnification
Lens mm
Telescope designs haven't changed that much (in the last few hundred years even; aside from the quality of optics and mirror polishing), but depending on the quality of the telescope you already have, you may only need to get some more lenses. Find out which diameter your focuser is (either 1.25" or 2") and buy a decent starter kit with various focal lengths.
If doing so, make sure you keep in mind the limits of the telescopes maximum resolving magnitude (I can't stress this enough because you could be wasting time/money buying eyepieces that will virtually do nothing because of what your telescope is capable of). If you call Orion, Zhumell, Celestron, or the like (any reputable optics manufacturer), I'm sure they'll ask all the necessary questions to get you on the right path.
Or better yet, tell us the make/model of what you have already and I'll be glad to help more too.