The easiest thing is to find a program (maybe at the library) that aims to teach phonics. If that's not possible, new readers need (in a nutshell) to learn the following:
The sounds for each of the vowels
The sounds for consonants (including hard and soft sounds for letters like "c" and "g")
The "blends" of any letters that are often teamed up with others to make completely different sounds. (st, sw, tr, tw, bl, br, etc.) Going through the consonants in the alphabet, and thinking of whether each is used in a blend can help you think of all the blends. Don't forget "th" and also the endings of words, like "ing".
Children can learn by starting with the simplest of sounds and seeing rhyming words (cat, hat, fat, mat, pat, etc.) When teaching a sound like "th", instead of rhyming let them them see several words, "this, that, through, thought, thing, etc." If they see that "th" appears in all those words they should make the connection.
The above are the most common, most basic, aspects to phonics. If a child learns that much, it isn't difficult to add a few "fancier" points later.