Playing as Left Midfielder Requires Great Power
Left midfielder is a position where you must be fit and have enough power to participate in both defense and offense. There are no differences between a left and right midfielder, beside the foot they receive the ball with.
Posing a good one versus one skill is a must if you want to play on this position, this because you will often need to get around your opposing left or right fullback and serve your teammates with quality passes.
During offensive attacks, your task is to stay wide and prove width. By using this strategy you will stretch out your opponent's defense and create more space for your teammates.
This will give them opportunity to receive the ball with little pressure, and also more time to figure out what to do next.
This will increase chances of scoring, so just because you don't have the ball it doesn't mean that you should stay still and pray that your teammates will score.
Without The Ball
In fact, your game without ball is probably more important than with the ball. This, because under a soccer match you will approximately have the ball under control at your feet under two minutes, which means that you must find something else to do under remaining 88 minutes.
With something to do I don't mean that you should sit and drink coffee while hoping for the best, when you don't have the ball. Instead you must try to involve yourself in the game and to help your teammates in defense.
This is a position that mean almost same thing as the mentioned right or left midfielder. The main difference is that a winger comparing with a left/right midfielder is very offensive almost in line with a forward.
As a winger you are supposed to play wide, dribble the ball forward and serve crosses to your teammates. Lionel Messi (Barcelona FC) is seen by many as the best winger in soccer today. His stamina, speed, and control of the ball make him perfect for the winger position.
Try to watch Messi while he plays on TV and practice on the same moves that he does. Notice also what he does before receiving the ball, if he participate in the defense, does he shot often from long/short distance or does he prefer to pass the ball to some better positioned teammate.
This position is more comprehensively covered comprehensively in my guide, The Ultimate Guide To Soccer Positions. If you are serious with your soccer career and want to become a better soccer player, make sure to take a closer look at this book.