Heroes Selection Screen
Before entering the battlefield, each player is required to select a hero. There any different roles and type of heroes in the game. Clueless player tend to choose their favorite heroes and locked in without thinking about the team composition. When they realized the team lack of damage/tanks/crowd controls/etc. it would be too late to change. Therefore, we will be discussing about how to pick their heroes and make a strong team composition. In casual and below Diamond ranked games, it's just blind picking. Team from both sides couldn't see each other picks and anyone can pick the champions they owned or on the free rotation.
First of all, it's never good to have TWO mages, TWO marksman or TWO junglers in a same game. If you have encountered such issue, advice your teammates to follow-up with the team composition and pick a better role. Each team need at least 2-3 front lines so the entire team doesn't get shut down in a few seconds.
Basic Team Composition: This team composition is formed based on most MOBA games such as League of Legends. They have a solo laner (Dark Slayer), Mid laner, Duo-Lane (Marksman and Support Abyssal Dragon) and a Jungler. It's fine to have such composition but the support has to roam a lot in order to let the marksman farm better and faster. It's very common in solo-queue games because the lack of communication.
Roamer Composition: Consists of three solo laners (Dark Slayer, Abyssal Dragon and Mid lane), a roamer and a jungler. The good thing about this composition is because three players get to solo their lane and gain the highest gold possible from the minion waves. The roamer should be a high-based damage or with decent crowd control such as Ormarr, Thane, Mina and Skud. The roamer's job is to pressure the enemy junglers and also the laners while protecting your jungler to farm in peace. They could even set up a gank with the jungler to have a 3-man tower dive. Your ally jungler should be a late gamer that can snowballs well and carry games such as Violet and Slimz. It's very common in a 2-3 man queue games. Since the nerf of Soulreaver, there would be lesser Marksman in the jungle.
Protect The King/Queen Composition: Having one really strong late game Marksman (Tel'Annas, Violet, etc.), sometimes another mage too and three-four tanky frontlines and support to protect the marksman. Also known as "Peeling Composition". The gameplay is to play around a marksman, protects him/her and preventing the enemy team from assassinating your marksman in teamfight. It requires really good team communication so that no one gets sloppy and letting the enemy assassins destroying your backlines. Mina or Thane + Chaugnar are usually played to peel for the marksman even better.
No Marksman Composition: Also known as a "diving composition". Relies on 2-3 powerful fighters/assassins to deal damage while ignoring the enemy tanks in teamfight. The goal is to destroy the enemies' backlines and the frontlines next. There would be no marksman in the team so it's not required to peel for the backlines. There are some reliable picks that doesn't need peeling such as Lauriel, Tulen and sometimes Joker. In the early game, they would just pick on lonely targets under the tower and tower diving them. Played mostly in a 5-man team, mostly in competitive games. Since the nerf of Soulreaver, I have no doubt that there will be more of this team composition.
Pick & Ban Phase
Pick & Ban Phase is only available to certain modes - Ranked Game Diamond & Above and Custom Game Tourney mode. Team from both sides can ban at least ONE hero, total of two heroes ban which can't be picked by the players to play in the battlefield. There is no "Blind-Pick" in the pick & ban phase, every player can see each other's picks including the enemies'.
It's important to ban the top tier heroes if your team doesn't get the first pick. Otherwise the enemies might pick the powerful heroes into their advantage. After 10 players picked their desired heroes, there would be some time where players could swap with each other if they owned both of the heroes. For example, Player 1 and 2 both owned Violet and Chaugnar, they can swap their heroes with each other before the game starts.
One side of the team (rotates randomly in ranked game, left side/blue team in Tourney Mode) gets the first pick of their heroes. The pick would goes like this- First Pick from your team > Second/Third Pick from your enemies > Fourth/Fifth from your team > and it goes on until all ten players have picked their hero.
Knowing the weakness your enemies
During pick & ban phase, it's important to pick accordingly to have counter against your enemies. For example, enemies has huge amount of crowd control in teamfights, your team could counter-pick with a Chaugnar to cleanse against the crowd controls. Enemy has too much late game heroes? Pick early game heroes and play aggressively to snowball! There are so much possibilities on how a team composition works. Only experience could tell on how to counters them properly.