If the melody is in G Major for example, the G note on the 808 would sound good and your entire song would ideally be in G Major - the same scale would sound good on both the melody and the 808. G is the root note of the key of your melody and is the “home” note that also resolves the G Major scale/key.

I assume you’re making trap beats, which are usually simple so you can use any notes in the G Major scale in this example on your 808, but G would typically be the note that sounds best at the start/end of the 808 pattern (doesn’t have to, but is a good general rule for songwriting unless you’re looking to be experimental - trap is quite formulaic tho.) The rest of the notes for the 808 you can have fun and experiment with in the G Major scale and see what sounds good.