Making chords is a very important part of music creation. In a Key Editor or Piano Roll editor available in most of DAW apps, it is really easy. I'll show you a example made in Cubase:
C Major Chord
As you can see, it is mostly a question of counting half steps.
D minor Chord
This are the main chords made with three notes (triads):
M : Major chord
m : minor chord
Aug : augmented chord
dim : diminished chord
When we add a fourth note (a seventh) we have a seventh chord. Based on the kind of base chord and the kind of seventh we have added , we have:
Maj7 : major seventh
mi7 : Minor seventh
7 : Dominant seventh
miMA7 : Minor-major seventh
dim7 : Diminished seventh
m7 (b5) : Minor 7 flat 5 chord
... So now -almost- the full picture!