The magnification of a telescope changes as the eyepiece is changed. Magnification can be calculated by dividing the focal length of your telescope by the focal length of the eyepiece.
Calculating magnification = Telescope's focal length / Eyepiece focal length
Always start with your lowest magnification eyepiece (longest focal length) and work upwards from there.
A 2x Barlow lens will double the magnification of whatever eyepiece you use with it while preserving its eye relief. For example: using a telescope with a 800 mm focal length with a 16 mm eyepiece will give you 50x magnification. Using the same telescope and eyepiece with a 2x Barlow lens will give 100x magnification. This would be the same magnification as a 800 mm telescope with a 8 mm eyepiece.