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.
Step 3: Tips
-Use a metronome! It isn't just for drummers. Using a metronome will encourage you to keep going when you make a mistake.
-Learn scales! You must practice scales every day. Not only do scales help you get familiar with your instrument, scales appear everywhere in music (eg: Joy to the World is just one big major scale at the beginning). Once you can play scales, you can recognize them in music and it makes sight-reading tons easier.
-Finding sight-reading material can be difficult, but be creative. Read out of a hymnal, a piano book, a band workbook, anything!
-Do not get discouraged when you make a mistake. It's always tough when you first get started, but once you get the hang of it it can be quite fun.
I hope you enjoy the music you make.