Picking for the team and not just yourself. How to create a team comp based off your team, and the importance of doing so?
All champions are unique and have different abilities, some champions go really well with others and some are better at fighting 1v1. When you are in champion select you have the choice to pick any champion you own, but normally will pick a champion that can fill a position that your teammates have not filled yet. Picking to fill a position for your team is just the first step in picking for your team, picking based off what your team was already picked will give you the that edge it may need in team fights later in the game.
Some champions may have some abilities that are similar to other champions but there kits are different and unique, making it so they are better with some champion and not as good with others. Example: "Amumu" and "Yasuo" is not as good as "Malphite" and "Yasuo" because they don't have abilities that complement each other. Knowing what champions complement each other comes with practice and knowing the abilities of all the champions in the game. You can also find good "comp" ideas by watching the pro players in tournaments, they know how important team fights are and seem to always inspire in making new ones.
When in champion select the best thing you can do is pick based off your team, but this does not mean play a champion you have no idea how to play just for the team comp. What may happen if you do so, is you will not be able to play the champion correctly, do to lack of experience and may cost you because of you don't know how to play the champion. Even champions that look easy may not be because all champions have a unique play style and you may not be accustomed to it. Try to pick for the team but also pick a champion you feel comfortable playing and know how to play.
The importance of picking for your team is critical for team fights, in some cases you can win the game base off the team comp. All lanes may lose but if you have good teamwork and a good team comp to back it up it may just change the outcome of the game later in the game. Communication in champion selection is key to building a great team comp and if done properly they may let you pick the role you wish to play.
Note: If you are playing rank the players that picks first are the player that are the highest in the pick order and the pick order is random. If you are the first to pick you may not be able to pick based on your team but you can pick with the team in mind, playing a champion that goes good in a comp.