The first step to improving at sight-reading is committing to consistent practice. A good starting point is to practice fifteen minutes on sight-reading every day.
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.