Many books are aimed at teaching you how to read basic music notation (see resources). "How to Read Music" by Roger Evans is a direct, easy-to-understand book that will teach you the basics of music notation. You'll learn treble and bass clef, note and rest values, how to identify bars of music and time signatures. The material is presented in a logical fashion that makes it easy for even a beginner to self-teach reading music. "Basic Music Theory: How to Read, Write, and Understand Written Music" by Jonathan Harnum is another highly recommended book that will teach you not only basic music reading, but more advanced musical theory as well. Also, consider buying a set of music flash cards. These are a good way to help you quickly recognize notes and other musical elements. You can purchase a set at just about any local music store.
Music software is ideal for learning to read music on your computer. Jayde Musica is an excellent piece of software for improving sight reading. You can play it online or download it, and while it isn't free, it never expires. The developers allow you to use it as long as you want and ask that you pay them what you believe it's worth. The software works like a game. You choose the instructional details you'd like and then identify notes as they move across the screen. You can set the difficulty level to easy, medium or hard. This software will quickly improve your ability to read by sight.
Another piece of software recommended for teaching you musical notes and other elements, as well as improving your sight reading ability, is Etude software. This software is an extensive course in reading and understanding note and rest values, treble and bass clef, key signatures and other musical notation elements. The software isn't free, but you can download it and give it a try before you buy.
The great thing about the Internet is the information available, in many cases free of charge. Music notation instruction is one of the subjects covered extensively, and free, online. Some websites rise above the rest, offering thorough basic music instruction arranged in a manner even beginners can work with (see resources). Some of the best websites for learning how to read music include Datadragon's "Introduction to Reading Music," MusicTheory.com and the "Standford Common Music Notation Guide." All of these methods offer information presented in logical order that can be used to teach yourself how to read music. If you feel you can't teach yourself, you can consider a local piano teacher who can teach you the basics. If you can't afford a music teacher, YouTube has several videos that instruct you on reading music. You can pause and rewind these videos until you master the information they contain.
The best way to learn to read music is to write it. Purchase blank sheet music from a music store or download some from the Internet. Write down what you learn to help reinforce it. For instance, when you learn that a whole note equals four beats in common time, a half note equals two and a quarter note equals one, get out a piece of music notation paper and practice filling bars of music with various combinations of these notes to make up four beats. When you learn about rests, put them down on paper. Every time you learn about a new musical element, writing it down will only help you understand it better.