As others are saying, you should definitely watch Clone Wars first. You can skip the movie. Honestly I'd recommend following one of those recommended viewing guides to dodge the crappy episodes, but if you can stomach them then you can just watch everything too.
The later seasons have some of the best Star Wars stories period imo.
Rebels is not nearly as good as Clone Wars, and the best episodes actually involve Clone Wars characters and tying up their loose ends.
EDIT:
Here, follow this:
And by follow it, I mean watch everything that is either highlighted OR an "Honorable Mention". Don't skip those.
Anything that is neither can be passed over no problem. If you really like the series you can go back and check them out, but it's mostly stuff like Jar Jar shit and much more exclusively kid-targeting episodes.
So basically, start with S1 Episode 2: Rising Malevolence.