Hi, Joe. Downbow on downbeat a good baseline rule, one which should absolutely be broken depending on phrasing. Also, depending on the style played, some rules don’t apply. Take a look at Kevin’s lessons, for instance, and you’ll notice that he often favors upbows on downbeats.
You can see opinions from our various instructors here: Fiddle Bowing Technique Lessons
That’s probably a good place to get some ground rules from which to build your own versions of tunes. Best of luck…you’re now tackling the fun stuff!