You just need to learn the basics of competitive battling, such as learning each Pokemon's particular strategy and study all their most-used movesets, which abilities/items to use, IVs, EVs, and natures.
Also, you need to study all the different tiers, and which Pokemon are housed in that tier based on their movepool, type coverage and strategies. There are several tiers in the competitive battling metagame such as Ubers, NU (NeverUsed), RU (RarelyUsed), UU (UnderUsed), OU (OverUsed), BL (Borderline) and LC (Little Cup).
Smogon is a recommended place to learn the basics, and you can also practice on battle simulators such as Pokemon Showdown. Also, team building is important since you need to take things like type coverage, strengths/weaknesses and more into factor. And you also have to decide if your team is going to based around a particular strategy, like Trick Room or certain weather conditions.
Also, PC has loads of experts in competitive battling so feel free to ask them if you need to learn more about the metagame and what you can improve on.