Choose a music theory online tutorial. There are a number of them available that cost nothing, and teach reading music effectively. The DataDragon Learn How to Read music is one of the best, most comprehensive online approaches to learning how to read and write music.
Bookmark and return to your online music tutorial on your schedule. Work at your own pace and use blank music paper to put into practice what you learn from each lesson. Simply reading instructions isn't enough. You'll learn better by practicing.
Begin with the five lines and four spaces on the music staff. Memorize the names of the notes and how to figure out the notes above and below the staff. This is also a good time to familiarize yourself with time signatures and clef signs. You'll want to learn the note names on both the treble and bass clefs. A good music tutorial website will categorize the information for you in an easy-to-understand format, arranged in a logical learning order.
Study note values next. A note value tells you how long to hold the notes you play. Examples of these include whole notes, half notes and quarter notes. You also want to learn the equal value rest symbols that go along with the note values. This is a good time to get familiar with the major and minor key signatures as well.
Study more advanced music theory next, such as how chords are built and how they progress within a given key. After this, you can expand your music reading knowledge to encompass symbols and other elements that are specific to the instrument that you play.