Sorry if this has already been posted somewhere...

I can't find a precise answer to this question: my story is set in the US, I use american spelling, but I'm not sure if I should use toward or towards? Among or amongst? Amid or amidst?
I couldn't find if the choice is related to UK or US english, or formal/ informal language. And if I use towards, does it mean I should use amongst, or not necessarily?

I'd appreciate it if you could help me out. Thanks