Ok, I'll help you with this stuff.
Breeding: This is probably the hardest part of making a competitive Pokemon. First off you need to know what IV's are. Here is a link to show you what IV's are.
Then you need to know how to get them. One of the easiest ways is to get a 6iv (6iv meaning it has 31 ivs in all six stats) Ditto. Now you may be wondering how you pass IV's down. When you breed Pokemon 2 random IV's from either parent can be passed down, but there is a few tricks to make it easier.
If you give either parent (doesn't matter which one) any of the power items it will pass down that parents IV in whatever stat the power item is for. Also if you give either parent a destiny knot it will pass down 5 ivs from either parent (not the one you gave the knot to).
So that is half the battle for breeding. The other half is Nature. Natures are a key part of competitive Pokemon. They will increase one stat by 10% and lower another by 10%. This isn't quite as hard as IV's though.
Nature's are completely random, except their is one way to keep a parents nature. You can give a parent an everstone and that parents nature will be passed down to the baby.
EV training: There are two different ways to EV train. You can use super training. You should be able to figure it out yourself, but the other is horde battles. This should explain it to you, but if you have any questions please ask! EDIT: Here is another link about EV training
Moveset: There are many different movesets you can use, but here are the 2 most used ones .
Tangrowth @ Assault Vest
EVs: 252 HP / 4 Def / 252 SpD
- Giga Drain
- Knock Off
- Rock Slide
Tangrowth @ Leftovers
EVs: 248 HP / 252 Def / 8 SpD
- Sleep Powder
- Giga Drain
- Knock Off
- Leech Seed
Abilities: These are pretty simple. You can just look at them and generally tell what abilities are better. But if you have questions about this or movesets (or items) you can look here
Items: For items you need to know the Pokemon. Greninja is a fast Pokemon but it doesn't have much bulk so leftovers aren't the best option on it, but a life orb or focus sahs work well. But Slowbro is a bulky Pokemon so leftovers work well on it. Also you need to plan your mega. If you have both slowbro and metagross on your team you need to choose what one you want to have its mega stone.
I hope this helps you with getting a competitive Pokemon. If you have any questions please ask in the comments and I will answer them.