Ed. Have a good look through the site you'll find all sorts of information you never thought you needed to use.
If you have a 12 inch dob then its focal length would be around 1500mm. To find the magnification you are using divide the focal length of the scope by the focal length of the eyepiece. eg 1500/10= 150X
Use the largers mm eyepiece to start with (especially when trying to line up your finderscope on a distant tree or telegraph pole) and then once you have found your planet of interest slip in the 10mm eyepiece. You will see Jupiter quite well at that magnification though a 2X barlow would be useful. A barlow multiplies the focal length of your scope. eg 2X barlow give you a 3000mm scope and you 10mm eyepiece gives you 300X (which in most conditions will be a bit much mag.)
Hope that helps you get on your way.
