Watch the Harmonizing a Melody online guitar lesson by Matthieu Brandt from Songwriting on Guitar
One of the issues you come across in writing a song is the chord progression you want to use for your melody. Finding chords for your melody can be a lenghthy process with a lot of choices.
Often this happens simultaniously with writing lyrics and melodies. Your ear will tell you what works and what you like. You could already have half a melody, maybe some chords and lyrics you want to throw on top. Each of the choices for chords you make influences the overall sound the melody has. In this section we'll look at what your options are and what would be a good workflow that will give you all the harmonic choices you need.
Remember: there aren't really any mistakes you can make, only choices.