Once you have a scale to work with, you can use the notes in that scale to build a chord progression.
The majority of western music revolves heavily around the 1st, 4th and 5th degrees of the scale. Below are two charts that you can use for reference when working in any major or minor key. Click the image to be taken to a new page where you can download them as a pdf for later use.
Here’s how to understand the charts:
The roman numerals from I to VII represent the notes or degrees of the scale from one to seven. The rows beneath tell you what the chord is for each degree of the scale. A capital letter alone represents a major chord, a capital letter with a small m is a minor chord, and a capital letter with a degree sign (º) is a diminished chord.
Remember: These chords are just a guide. They will be particularly helpful if you are stuck on writing a progression or melody, but using your ear is always key. If you’re happy with a progression outside one of these scales, ditch them. Do what sounds good to you. Remember, there are an infinite number of possible scales, these are just common to western music.