Begin with the alphabet song. This is the popular sing-song ABCs that most Americans learned while growing up. It's the song that ends with "now I know my ABCs, next time won't you sing with me?" If you aren't already familiar with this cultural favorite, you can learn it on a variety of websites. Some of these are listed at the end of the article.
Consider buying children's alphabet CDs. These CDs will include more than the basic ABC song and begin to familiarize children with the different sounds the letters make. Some funs one are "Here Come the ABCs" by They Might Be Giants, "Sesame Street: All Star Alphabet" and "This is Alphabeat" by Alphabeat.
Practice first-letter sounds with the child. After they have listened to some of the CDs and you have told them some first-letter sounds, try to get them to guess sounds. For example, say to the child "You want me to read you a book? What does 'book' start with? Listen: b-b-book."
Be patient with the child. At this age, children are not scholars and will not learn best by forced practice. When they can't figure something out, it's OK to tell them; this is how they learn. Generally, preschoolers will be excited to learn, and telling them what letters words start with or what letter comes next isn't cheating, it's giving them information.
Show the child how to write letters. Use larger crayons for their small hands, and allow them plenty of space on the paper. They will not be able to form small, neat letters. You want them to get approximations of the letter and practice the task of writing. Do not expect perfect, even legible letters at first. It's the practice of writing that will get them to write with more control.