In competitive Pokemon game, It is important to plan you team strategically. There are 3 role that a Trainer should consider at is attacker, tank, and support. The thing to consider for picking your  Pokemon for your team is stats, IV, EV, ability, nature, and skill pool. 

    Stats refer to the information of a Pokemon hp, attack, defense, special attack, special defense ,and speed value.

    IV refers to individuals value and it determines how strong each individual Pokemon will be. IV usually between 0 to 31, it tell you how the state will increase when leveling.

    EV refer to efforts value.  Trainer could increase EV of a Pokemon when defeat another Pokemon. The max for EV is 252.

    Ability is similar to passive.

    Nature determine what stats is the Pokemon strong and weak at.

    Skill pool refer to how many strong skill that a Pokemon can access.

  For Pokemon that considered to be an attack, It should have out standing attack or special attack stats, 31 IV on attack and speed, 252 EV for attack and speed, ability that could boost speed or attack, nature that increase attack or speed, and access to powerful attack move.