It's meaningless to me. He did a favor for me.

Ok so generally "for" is to indicate a purpose if something while "to" is to indicate a destination. I believe for your example if want to be precisely correct then it depends on the broader context of what is being discussed and how said subject is meaningless to you... however you would be understood just fine using either one in casual conversation. Also when your asking for help on correcting your "sentences" I only saw one sentence, I dont know if this was just simple typing mistake, but when you add a "s" to words you make them plural to indicate more than one. Hope this helps, Im new here, native english speaker just downloaded this app looking to start my own language journey :)