Because reading glasses are sold over-the-counter and you don't need to worry about measuring your pupillary distance to wear them, once you find a design you like, there's a good chance that finding your prescription will be a breeze.
Your accurate prescription, including the distance between your pupils so that the prescription is lined up correctly for your proportions in the lenses (you'll need to find your pupillary distance yourself if you are ordering online).
The optician will be able to measure your pupillary distance (which is especially important to get correct if you're going to be ordering bifocals of any kind) and let you try on frames.
You'll be able to print out a true-to-size image of the frame you're considering so that you can compare it to the pupillary distance.
All you need to know is your prescription and your pupillary distance (see The Sight Site to find out more on how to measure that).