We have a lot more chord choices when writing songs in minor keys than we do if we're writing in a major key. This is because we compile two scales to create these chord choices; both the (ascending version of the) melodic minor, and the aeolian (natural) minor scale.It is not necessary to know or understand these scales in order to write good songs. What you need to summarize (and memorize) from the above illustration is when writing in a minor key, chords can be found starting on the root (minor), the 2nd (diminished or minor), the b3rd (major or augmented), the 4th (minor or major), the 5th (minor or major), the b6th (major), the 6th (diminished), the b7th (major), and the 7th(diminished) of the key you're in. So, when writing a song which stays in the key of E minor, we could use some or all of the following chords: Emin, F#dim, F#min, Gmaj, Gaug, Amin, Amaj, Bmin, Bmaj, Cmaj, C#dim, Dmaj, and D#dim.
Phew! Lots of stuff to worry and think about. You might want to keep this in mind too: in most "popular" music, diminished and augmented chords really don't get used a whole lot. So if the above list looks daunting, try sticking to the plain major and minor chords for now.
In many traditional harmony books, you'll see the above series of chords, accompanied by a diagram that illustrates "acceptable" progressions of these series of chords (eg. V chord can go to i, or to bVI, etc). I have chosen not to include such a list, as I find it to be rather restrictive. Try combining various chords from the above illustration of the chords in a minor key, and decide for yourself which sequences you do, and don't like, and develop your own "rules".
Next, we'll analyze some great songs to find out what makes them tick.