Sight-reading is the musician's ability to play musical scores whether they have seen them before or not. The ability to sight-read is based on familiarity with the principals of musical structure as well as knowledge of musical notation. Some grasp of music theory usually precedes effective sight-reading. In fact, a helpful way to understand sight-reading is to think of it as a way of applying the theory behind music. On your way to acquiring sight-reading ability, you will become familiar with musical forms and styles, intervals, keys, scales, chords, and melodic structure. But you don't need to learn all of these things at once or even at an equal pace.
Think of the process of learning to sight-read in terms of how you learned to read words in your primary language. First you learned the letters of the alphabet, then the sounds that the individual letters made. Then you began to notice that certain letter combinations were appearing repeatedly. You learned to read simple words, then simple sentences. Soon you had facility with many words, and you began reading longer sentences and acquiring a larger vocabulary as well as a better understanding of grammar and punctuation.
Music has its own building blocks that can be reduced to the simplicity of elements that operate like an alphabet, letters, words, phrases, sentences, grammar, and punctuation—each of which you became familiar with over time. Now think of the alphabet as only A, B, C, D, E, F, and G. Imagine that the letters are individual notes on staves—the sets of 5 lines on music staff paper. Chords are similar to simple words in this analogy. You create "phrases" and "sentences" by combining chords and notes in a logical series. "Grammar" and "punctuation" can be seen as analogous to music's key signatures and timing symbols.
You eventually developed the ability to read the great literature of the world having started with only the simplest building blocks of written language. Remember how exciting it was to realize that you could read? Now imagine how exciting it will be to read music in a similar way and to be able to express yourself through music that you enjoy playing.
If you have tried to learn to sight-read before without great success, you may just need to approach it from a different direction. For example, most of us learned to speak before we learned how to read. Probably most of us had also learned to sing some songs before we even knew about musical notation.
Imagine setting yourself an initial goal of reading chord symbols and rhythmic notation based on songs that you already know. The music that we encounter daily is easily broken down into chords and rhythms. Most of these songs are readily available on the internet and in books containing musical scores.
Using these resources, someone learning to sight-read is able to hear, see, and play music that they are already familiar with and enjoy. Being able to sing or hum along to familiar rhythms and chord progressions takes away the frustration of learning to read notes from scratch and then attempting to find them on your instrument.
Just as you learned to associate marks on paper with words that you already knew, by learning to sight-read through playing songs you already know, the musical marks on the score paper—music notation—will become increasingly familiar to you. You'll learn to recognize patterns that you've seen before when you encounter them in new contexts.
You'll be putting simple elements together in increasingly complex structures developing greater skills, and greater satisfaction as you go, and becoming a better musician in the process.
There are just as many methods of teaching—and learning—music as there are different styles of learning among music students. Find a method that works for you, and stick with it, and you will be sight-reading proficiently before you know it. Here's what you can look forward to as your sight-reading skill improves: the ability to learn new music easier; the ability to take on a broader range of musical pieces; and the ability to be a more versatile musician who is a welcome performer in many more settings.