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.

Always start with your lowest magnification (longest focal length) eyepiece 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 900mm focal length with a 20mm eyepiece will give you 45x magnification. Using the same telescope and eyepiece with a 2x Barlow lens will give 90x magnification. This would be the same magnification as a 900mm telescope with a 10mm eyepiece.

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