To buy eyeglasses online,you should get your eyes checked at the local optometrist or optician first and make sure to take your prescription data with you, and then go browse and try on glasses at the nearest store. Don't be shy, go ahead and try anything your heart desires, even the most expensive models.
When you find a few favorites, write down the makes and models, or maybe even snap a few quick pictures so you remember the look you wanted. Then have a look through online eye glasses vendors for the same makes and models, or at worst a different brand with a similar look. You may be pleasantly surprised by the savings.