First of all, you need to choose which role you will play the most, for example, my main Role is Jungle, but also you need to know play two roles, because you have to choose the secondary role too(My secondary role is Top)

After choosing your main role, you need to choose which champions you will play
For example:My main role is Jungle and I main Zac, Warwick, Sejuani, for top lane(my secundary role) I chose Renekton, Wukong, Galio.
You need to limit your champion pool for ceratin Role.

You need to learn that you can win game in champion select.
First of all you need to watch on AP/AD balance of your team. For example, if your team is full AD, and you have last pick with ADC best option is to pick something like Corki, who have a lot of AP damage in his kit.
Also, usually u will need tank to initate fights, so having tank in team is always good.

This can be really frustrating for jungler, (For example: I picked TeamFight champions like Amumu, Sejuani, and enemy jungler pick early game jungler like Kha'Zix or Lee Sin, of course they made more early game preause, and my teammates just give up.
Point was, you need to learn which part of game your team will be better then enemy team, of course best option is to have champions with early game potencial, but also some carries which will be able to carry late game.
If you have junglers like Lee Sin or Kha'Zix you will, usualy stomp early game, but they easily can be outscaled in late game.