Step 1: Commit and learn the basics
Before you can start sight-reading through the Mozart symphonies, though, you must know the basics of reading music. Kevin Meixner presents the basics in a written article format in this website, and Ricci Adams' Musictheory.net helped me to test out of Theory I and II in my freshman year of college. But perhaps the best source is someone you know who is an experienced musician: your band director, piano teacher, and choir director will all be more than happy to talk with you about music.
Step 2: How to practice
Always take the piece at a tempo that is comfortable for you and do not stop playing, even if you make a mistake. If you are in an audition, the judge will not want you to stop playing just because you've messed up.