If this is the first time you buy a pair of glasses online, you can have your eyesight checked by an eye doctor and then get a prescription which can tell you what are your degrees and PD. Sometimes an optician can tell you which size is best for you and which kind of frame can fit your face shape. But nowadays, many opticians just sell their products and know little about this, while they will let you have a try. If you can find a pair of glasses that fit for your face shape, you can buy that. If you want to buy the glasses online, you can remember the size of this pair of glasses and place an order of the same glasses online. Never let the optician know that you will not buy their glasses before getting all the information you want in their stores. Good luck!