To those who have never learned music theory, looking at a sheet of musical notes is as unintelligible as another language. The lines and symbols seem completely disconnected from the amazing music produced by experienced players when reading them. However they’re actually very simple to understand, and follow a regular pattern without any of the exceptions or quirks found in languages. The key is to start simple and then build your knowledge up to more and more complicated musical sheets.
One of the best ways to learn is with a tutor, and you can find extremely knowledgeable ones on Youtube, such as this Free Music Theory series. Using just a whiteboard and pen, the instructor teaches you the basics of musical theory – without you needing to play an instrument at all. Starting with the basic structure of a music sheet, he explains how the different notes can be arranged. Better yet, he encourages watchers to make notes and annotations so that you end up with something solid in front of you, rather than just passively watching someone demonstrate.
There are a range of videos that take you through the basics to more complicated theories, so you can start wherever your knowledge ends. You may also just want to watch to touch up the knowledge you already have. They’re especially useful to those who have never learned formally, and may be missing large parts of practical understanding. Even for those who have never touched an instrument in their life, it’s a great background to help you get started with your learning. It can also help you appreciate all the different music you hear day to day, as you are able to recognize its complexity and depth.
Wikipedia also provides a superb introductory page about music theory.