Ok well, competitive Pokemon is an incredibly complicated topic that can't be fully explained in one post so I will explain what I consider the two fundamentals to be, IV's and EV's. I highly recommend you watch Verlisify's channel on Youtube since it contains tons of guides. IV's and Ev's both relate to the stats of a Pokemon. IV's are a set of base stats a Pokemon is born with and can never change through training, a Pokemon can have up to 31 IV points per stat and each IV point is one stat point. EV points are developed via training. Think of them like allocating points into different stats in an RPG like Dragon Age Origins. Whenever you defeat a Pokemon a certain amount of EV points are given to the Pokemon, usually one or two. Four EV points equals one stat point in a certain stat. When it comes to competitive battles it's important to allocate these EV points wisely because you can only have up to 252 of them. For example you don't give a high damage sweeper Pokemon EV's into HP since it's not designed to take a hit, you would instead allocate EV's into more attack points. Like I said this is way to long of a discussion for one post so your probably going to want to surf around on Youtube.