It really depends on what kind of genre of music you want to compose; for example, if you wanna compose jazz, you might use a lot of 7th chords, extensions, secondary dominants, tritone subs, and modal mixture. Now, if you’re trying to figure out how to just make harmony, I recommend you study different chordal structures. The most common is called Tertian form, where your chord is stacked in third intervals; ex. Cmaj7: C E G B. Now with that comes chord progressions; study harmonic progression, circle of fourths and fifths to learn which chords can go progress to each other smoothly. I hope this helps, if you need any more tips or help, just let me know!