How do I separate many devices controlled by one switch?
That sounds like your standard basic electrical bodge job, where it once worked, and then some homeowner or handyman got in there and mucked it up good-n-plenty.  God knows what else could be wrong in there. Or what else that menace has done! 

I gather all this sparky stuff is new to you.  You may be better off bringing in a pro to correct that blunder and search for other flaws. They have a fair chance of correcting a dangerous situation.  

If you want to take it on yourself, good, but take your time (you don't want to become that menace).  Learning electrical is a wide mountain (but not a steep one). I'd hit the library and look at electrical books (adult nonfiction, section 621.3192) and browse a few and find one that speaks your language. Then indulge in reading most of it. That'll establish well-rounded knowledge on the subject, something Google is awful at... and then we or Google can fill in the gaps.