Hi, welcome.
EV's, IV's and Breeding
To start off, you'll need to know what IV's and EV's are. In a basic overview, IV's are individual values, ranging from 1-31 In the stats HP/Atk/Def/SpAtk/Spdef/Speed. EVs are basically extra points in the stats you want ^-^
As for Natures, Let's say you have a frail sweeper. Would you want the frail sweeper to have Spdef, or more speed so It can kill faster? Natures boost one stat and lower another, It can sometimes, save you a game, if you outspeed a pokemon, or barely make a Pokemon faint.
Pokemon are mainly divided into these 3 basic categories;
Wall
Support
Sweeper
EV Training: You can find a guide here
IV's and breeding: You can find extra details here
Natures: You can find a list of Natures and what they do here
Synergy
Next, is Synergy. Before you take a random bunch of Pokemon and make a team, you have to note the Weaknesses they have. You don't want your whole team to get swept by an earthquake, ya know?
For example, If I had an Umbreon on my team. Umbreon is weak to Fight, Bug and Fairy, so I would need Pokemon that resist it or are immune to some weaknesses like Chandelure or Skarmory.
Synergy: You can find more information here
Movesets
Movesets are important, so you can set up/ sweep efficiently. You want to give your Sweepers coverage moves so they can at least do neutral damage. Also, it's important to make sure, that if your Pokemon is a physical attacker, it has physical moves. If you're going to make it a mixed sweeper, make sure it has a attack stat that can do damage. >.<
Predicting
Your team can be virtually perfect, but if you don't know how to predict, then you're screwed
Predicting is basically guessing what the opponent is going to do, It's all mind games that are hard to explain >.<
You can find more information on Predicting here.
My Recommendations \ : D /
When making a team, I would recommend this;
- Have a Volt-Switcher, U-turner/Baton-Passer/ Ghost type so you won't be walled by Shadow Tag/ or Mean Look.
- Have a Special Wall and Physical Wall that can absorb easily predictable attacks.
- Have at least 1 Physical Sweeper and 1 Special Sweeper
Have an Espeon because Magic Bounce is OP
- Make sure you have a flying pokemon
- There's more... I'll edit when I remember later
Eek >.< Ali already replied ;-;