Pick a melodic range for the song. You'll need to know your vocalist or instrumentalist's range before choosing a key signature.
Choose the key for your song. First, decide if you want it to be major or minor. Second, pick the key that fits your singer's range.
Listen to songs you admire and learn their chords. This is the best way to become familiar with the standard I-IV-V progression that's the backbone of tonal music.
Write a basic progression that starts with the key's main major chord and leads to the key's dominant chord. Using the key of C-Major as an example, these would be the C and G major chords. This also works for minor keys, as well.
Experiment with the in-between chords. Add your own embellishments to the I-IV-V progression by including augmented, diminished and 7th chords. These give your song its own unique flavor.
Use chords like the major-7th (i.e. C-E-G-B) to give your song a jazzy flavor. For a darker tint, throw in a minor-IV (i.e. F-Ab-C) before returning to the tonic chord (the key's main major chord).