Learn music theory. Key signatures, circle of fifths, rhythms and anything notated in the music should be understood. The more you learn, the more you'll understand about music.
Select an easy piece of music based on your skill level.
Examine your music. Notice the key signature, time signature patterns and any changes.
Mark your music with a pencil. If you notice anything in the music that might throw you off, mark it so you can anticipate that spot when you're sight-reading.
Play slowly. Do not stop. If you make mistakes, keep going. If you find that you cannot make it through the piece, switch to an easier piece or play at a slower tempo.
Practice other selections in the same manner until you have practiced for at least 30 minutes.
Practice daily. With practice, you will be able to sight-read faster.