Simply because you're new you're also bad at the game. Playing with any bad players will significantly decrease your chances of winning.
As you develop your skills you will learn how to instruct others to properly target champions who need to be focused and you'll learn your place in a teamfight.
As a veteran player I can see the flow of a game. Levels 1-3 differ from 3-6. After 6 the game changes a lot. Once you get two points in your ultimate you're in mid-game. After 3 point in your ultimate you're officially in the late game and the game can be decided by your ability to team-fight. Nothing matters except your team's ability to teamfight and secure objectives. If your team can team-fight but can't take objectives, you will probably lose if the enemy team gets an ACE and takes objectives better than your team.
I suggest ARAM to develop your team-fight focus and sense of direction. Summoner's Rift is basically ARAM but split into 3 lanes with a jungle. It is extremely boring. ARAM is much more fun and what the game is actually about.