Can someone help me build a competitve team? I'm trying to make one, but all of guides I've found don't explain it very well...
FWG {Fire-Water-Grass} cores are a good start. Also, the structure of your team should be definable -- it shouldn't just be a bunch of strong looking Pokemon who work well alone. For example, Dragonite is a good Pokemon, and so is Goodra. But unless your team revolves around those two as a core, their utility for the team will just be stacked {because they are just bulky dragons meant to take hits and dish out powerful ones} -- limiting slots for other Pokemon who can fulfill a more solid performance as a team and in the battlefield.
Your personal skill level also plays a part in team building -- do you prefer stall {Pokemon who have enormous defenses who support and use passive damage (Toxic, entry hazards) as a win condition}, Bulky Offense {Pokemon who can take hits who can retaliate quite powerfully}, or Hyper Offensive (Fast, usually frail, Pokemon who exhibit constant offensive pressure to the opponent}? I am one who enjoys any and all types of Offense -- but Bulky Offense is a tad more appealing for me.
Guides all over the internet vary -- but I find
Smogon as the most reliable one. I should warn
you that some of their in-depth analyses on certain fields (Pokemon, items, training guieds) may be difficult to understand -- so I advice you to take the learning process slow and steady. Don't skim as you might miss some pretty vital information.
Now, for the Pokemon. I assume you're not playing in Smogon's tiering system as anything, so the first thing I suggest you to do is add Mega Kangaskhan. It's seriously strong as crap holy sh**.
But in all seriousness, you should choose a Pokemon (or a core) you would like to add in your team,
revolve your teammates around that core. And, the most important task,
test it. You're
bound to find holes in your team, which can be patched as soon as you gain experience. Not even the strongest player, nor the best teambuilder, can make a perfect team in one go. It involves a process of testing and tweaking. Not only that, but you should also keep in mind the playstyle you choose. Greninja doesn't fit well in a team full of Bulky Support Pokemon, but Aegislash is a versatile Pokemon that can work well in each individual team structure. Every Pokemon is different, so I advice you to try them all out and see which Pokemon fits what role, etc.
If you don't want to waste your time breeding on Pokemon that you're unsure of using,
This site can give you a jump start. It can help you choose the Pokemon you would like to use for battle (no training necessary), and it can also hone your skills in team building and battling in general.
Another site I'd like you to visit is
the Smogon RMT (Rate-my-team) page. There, you can see why people choose their Pokemon in a team, and you can see professional team raters criticize them. It really gives you a good viewpoint on the current metagame, so I suggest you to check this site out. You can also copy their teams to play on Showdown! , if you're too lazy to team build.
Hope I help!

If you have any questions please ask me.
I'm writing this really long post for the TBT Bells and for a good competitive Pokemon community in TBT, haha.