Test out a few to see which one works best. Hold a book or magazine 14 to 16 inches away from your face, and see if the glasses make the print easier to read. You might need two different strengths one pair for very close reading and one for middle distances like your computer screen.
Choose the weakest (lowest number) glasses that will work for you. Keep in mind that these glasses might not fit you as well as prescription glasses. They also don't correct astigmatism (a common condition that causes blurred vision), and they can't be adjusted if you need a different strength in each eye.
If your close vision is blurry, make an appointment with your eye doctor for an exam. He can recommend reading glass strength or write a prescription based on your exam. And it's always a good idea to get checked for other eye issues.