Memorize the most commonly used guitar chords. Most songs are built around chord progressions, so a good knowledge of chords will help you learn songs on the guitar. Start with the major, minor and major seventh chords before moving on to more complex chords like suspended, augmented and major seventh chords. Purchasing a chord book for reference can be a wonderful tool to guide you when you are learning, as well as for the experienced player, though there are free online resources that can show you how to play chords.
Learn the most commonly used guitar scales. Guitar scales are often used to build guitar solos, lead guitar parts or to map chord progressions. The most common scales are the major, minor, pentatonic, minor pentatonic and the blues scales. Purchasing a reference book that shows you all of the positions of the different scales is recommended for both beginners and advanced players. Many public and school libraries have books with this information, and there are online resources that can be accessed for free as well.
Purchase a book of guitar tablature or sheet music of songs that you have recordings of. While sheet music is more expressive than guitar tablature, it is more difficult to read, and often unnecessary for rock or blues musicians. Purchasing music that you have recordings of allows you to play along with the recording as you learn the song to see if you are making mistakes.
To begin, practice any song you want to learn at a slower tempo than the original. When learning a song, particularly one that is difficult, you have to crawl before you can walk. Start with the metronome set at half the tempo of the original, and make it faster by 6 to 10 beats per minute as you become more comfortable with the song until you are playing at the speed of the original.
Practice difficult songs at least one hour every day. A big part of learning to play a difficult song is developing muscle memory through repetition, so if you do not practice regularly, it will be very difficult to master the song.