If you take the major scale (Let's use Key of C for example - C, D, E, F, G, A, B, C and added a 3rd and a fifth above each note you would have the chords that belong to that key C major, D minor, E minor, F major, G major, A minor, B diminished, and C major again. Now give the scale the roman numerals so that you can use any key. I, II, III, IV, V, VI, VII and VIII = I. Seven different chords in each key (not counting key changes or secondary dominants - another whole topic) OK now these chords need to follow a pattern or order. The I chord can go to any other chord but once there, there is a specific formula to return to I. Furthest out is III, from there you have to go to VI then you have a choice of II or IV. Next step is V or VII then back to one. You don't have to go to III. I could go to VI then II then V then I. There are lots of other rules that can enhance the progression but this will give you a basic working of chords.