When I want to change keys, I have the choice between 11 different keys. How do I know which key I want to change into?
don't worry about that. simply listen to the song all up to its current point, and imagine what you want the next part to sound like. then find out what those notes and chords were. then the analysts (maybe you, maybe someone else, ...) can try to figure out what key they went to. sometimes people may even disagree on what key it was, because when you change keys you have opened the door to change, and sometimes you may be roaming the hallways instead of going explicitly into 1 room. or in some cases you may have entered a room but through the side door, which can cause potential disagreements about which key it was. but it still sounds the way it sounds so just pay attention to what sound you want. unless you are making music primarily for a wall decoration, in which case you should use B# major, to wow your guests