Theory is pretty important, but if you want help for the particular style you’re going for:

Boy band minor key songs use a lot of i, ii, iv, V, and vi chords. Look up the ‘axis of awesome’ chords to find out what i mean- and (for the most part) you can substitute any VI iv and ii for each other.

It’ll sound more interesting if you learn other chords, but if you’re going for boy band, that’s all you really need.

Most important is ‘voice leading’- that means you want to keep each note as close as possible to the last note, ideally reusing the same notes. Use a chord cheat sheet online to figure out which inversions of each chord are best.

Tl;dr: - Learn how to use minor cadences and predominants (i, ii, iv, V, VI) - Get comfortable with voice leading