In contemporary music there are some keys that are very popular and commonly used. I will talk about some of those keys and their chord progressions and how they can be used in song writing.
Some of the very popular keys in modern music would be G, C, D, A, Am and Em. If you study the contemporary music you would see that majority of the songs actually belong to either of the above mentioned keys. Lets talk about the regular chords that we have in these keys
For G there is only one sharp which is F# and the chords are
G, Am, Bm, C, D, Em and F#dim
Em is the related minor key of G so Em has the same chords as G major
In the key of C we have no sharp and the chords are
C, Dm, Em, F, G, Am and Bdim
Where Am is the related minor key
In D we have two sharps F# and C# and the chords are
D, Em, F#m, G, A, Bm, C#dim
With Bm as the related minor key
In A we have F#, C# and G# and the chords are
A, Bm, C#m, D, E, F#m, G#dim
And F#m is the related minor key
Now if you want to write a song say for example in the key of C then you can use the listed chords since they will all sound good with C. You can also switch between the key of C and the key of A minor since they are related. This will give you more dimensions in song writing and also will help you when it comes to improvising.
