Welcome to Smogon, Aristo! I see you're here for competitive play, so you've come to the right place! I recommend that you start with ORAS, because it's still fresh, That's the metagame that most players are playing in, so it'll be good to catch up with the current events! A good plan for becoming a competitive player is to join th community and be active. Don't forget to ask questions and once you're very knowledgeable, you can also give tips! Chat on the forums here. If you have questions, try the "Simple Questions" threads or the "Have a Question? Ask it Here!" thread. On PS!, visit the tier rooms and Competitive Tutoring for various battle tipes or teambuilding help.
I hope you have a great battling experience with Smogon!