One of the best ways to become familiar with chords in all keys is to take a simple progression and transpose it to all of the keys. This may be slow at first, but the more you do it, the easier it gets. Here are some common progressions to learn and transpose. Remember to practice them in different ways, e.g., as block chords and also arpeggios.
As you can see, just as there are major and minor scales, there are also major and minor chords. There are also many other types of chords all of which have different names and sounds.
Each type has a different combination of notes and a specific chord formula. If you look at most sheet music you will see chord symbols written above the melody, symbols such as C, Am, Bb7, E7#9, etc.
All these symbols describe chords which have different types of sounds. The reason a certain set of chords works well with a particular melody is that both the melody and the chords are made up of notes from the same key.
When putting a melody and chords together, the basic principle is that at the beginning of each bar or where there is a long sustained note, or any time there is a new chord, one of the notes of the chord played at that point should be the same note that is used in the melody at that point. E.g., if the melody has a G long note at the beginning of a bar, any chord used at that point should contain a G note.
This doesn’t mean it has to be a G chord, it could be any chord which has G as one of its notes. E.g., a C chord or an E minor chord or an A7 chord all contain the note G. Understanding chords can be quite difficult at first and is a lot easier if you relate it to an instrument capable of playing chords (e.g., guitar or keyboard).
Here is a final song which uses almost everything you have learned. It has two verses: the first expressing a longing for a lost love, and the second angrily rejecting the ex partner. Listen to the example and notice how the singer delivers each verse in a totally different way.
This is partly done with dynamics, partly with range (verse 2 is sung an octave higher) and partly with the way the words are expressed. Keep these things in mind when approaching any new song and you will be able to sing in a way that immediately communicates itself to your audience. Happy singing.