The strength of your reading glasses should be determined by an eye doctor or an eye care professional. Recommended magnification is dependent on how nearsighted you are.
A good rule of thumb is to use the lowest magnification with which you can comfortably see, and work upward if you need. Low strength reading glasses (+1.00 or +1.25) are enough for some people, while others sometimes need to go above a +2.00 to read words clearly on a page.