Playing competitively is not very different from applying the basic principles of battling casually, except this time you're gonna have to focus the finer details of team building. Since you've never played competitively before, I would recommend reading
PC's introduction to competitive battling, as well as
Smogon's Introduction to Competitive Pokemon. While these articles seem outdated, a lot of the principles of competitive Pokemon hasn't really changed over the past year. Once you start venturing into competitive, you will soon realize that you'd have to sacrifice a lot of personal preferences if you want to get anywhere. Of course, it goes without saying that you should be able to balance out your preferences with the current demands of whatever metagame you're trying to play. For example, since Gen 7 is still in its infancy stage, playing around with literally whatever you want will actually help you gain knowledge of what the future might hold.
I know this isn't really specific help, but diving into competitive without really knowing what you're doing doesn't make it any fun. You're going to have to accept the fact that almost everyone around you will know substantially more, and you're going to have to adjust to that fact. Familiarize yourself with
some common terms used by competitive players, read some
RMT threads, specifically ones that look interesting to you (for example, you should look at what people are using with Mimikyu) and understand why people are doing what they're doing (or why other people are telling them to stop doing what they're doing and do another thing instead). Build a team, either from scratch or by copying other people's (just remember not to take credit of other people's ideas!), and get into actual battling with the clear goal of learning, not winning. Every game is a win if you've learned something, after all. :p
tl;dr familiarize yourself with the principles of competitive Pokemon before actually partaking in it and everything else should follow smoothly.
Okay that ended up way too long and kinda cheesy, but I hope you at least take something from what I've said.
