Make certain that you clearly understand the basics of note reading and counting before you begin. Review the lines and spaces and note values. Review the time signatures, key signatures and rests. You can't read music until you know the basics. If you need extra practice, get a note speller or theory book. Practice singing melody lines and clapping rhythms.
Study scales and arpeggios. Whether you sing or play an instrument, these are the building blocks of melody and harmony. Memorize these basics in all 24 keys, and you will see the patterns in your music. Study harmony to gain knowledge of chords, especially if you play an instrument like the piano.
Begin sight-reading practice. Choose music at your level that you have never studied, and preview it before you begin to sing or play. Take note of the key signature, time signature and tempo markings. Look through the entire piece, noticing large divisions and expression markings. Don't forget to look for repeats.
Sight-read the music by singing or playing straight through. Pretend you are following a director, and play at a tempo that you can maintain without stopping.
Continue sight-reading practice consistently every day. Keep a stack of new music handy for this purpose. Over time, you will make great strides with even 10 minutes a day.
Reinforce your daily sight-reading with mental practice. Try reading some of your music silently, hearing it in your head. Check yourself later at your instrument if you like.
Get extra practice by joining singing or instrumental groups, volunteering as an accompanist or joining jam sessions. Working with ensembles or groups will improve you sight-reading even faster. Be consistent with your study of sight reading, and your progress will amaze you.