Learn how to combine with the outside backs and center mid. Also the forward on your side if you are playing with 2 or 3 forwards. Learn how to serve crosses with both feet, this allows you to cutback if your stronger foot is marked well. Practice corsses alot and learn how to place them where you want instead of just sending them to the box. If you can master this and learn to lead a striker, you will be a gem to your team. Oh yeah, get as fit as you've ever been, remember you have to get back and defend.
Will your full backs be over lapping, or do they not over lap you? If they do then you will need to try and link up with them as much as possible, if not then you're going to have to use the line well or cut inside a lot, variation is key. Opposition players will learn very quickly if you are just hogging the line or cutting in all the time. Like the other poster said you need to be extremely bloody fit, unbelievably so, if you want to do the job well. Try to exploit the full backs of the opposition team and identify their weaknessses and strengths as early as possible then you can decide the type of style you want to play in that game. Also if you play a formation which isn't that narrow, so in this instance a 4-4-2, then try to hog the line as much as you can, and stay as far wide as possible, this will stretch the play and create more space for your team mates. If you are a converted winger then hogging the line can be extremely hard to keep up, as you will always instinctivly cut inside, but try and teach yourself, depending on the formation and how your team sets up to stay as wide as possible. Of course the obvious other things are pace and crossing. Like the other poster said try to build up your weaker foot so you're not that uncomfortable whipping in a ball with your weaker foot, this will obviously give you more options and give the opposition teams more of a headache.
When attacking with the ball, being able to vary your style of attack is key. You're best off if the defender doesn't know whether you're going to run to the endline, cut back and cross with your other foot, cut inside, or play an early cross. When putting the ball into the box, try to play it to either near post or far post - a cross into the center of the goalmouth often gets picked off by the keeper. Driven crosses are much harder to defend than floated ones, you might even cause an own-goal by whipping in a hard cross on a flat trajectory right at head level. Doubly true if you're at the endline and pulling the cross backward.
Not sure what you mean. The question is very general so I will have to give a very general answer. The best midfielders are the best all around players. For most players, there are three areas that can be improved: your fitness, your touch, and your weak foot. Improving your fitness allows you to cover more ground and therefore influence more play. Defense is just as important as offense. During a game you will find yourself changing roles constantly as the game flows: running into a forward position, supporting in the midfield, or tracking back into the defense. No one runs more than a good wing-half. Improving your touch. Obviously being able to pass is important, but not enough emphasis is put on collecting and trapping skills which will allow you to make a good first touch. Getting a good first touch is very important. Your weak foot. Ideally, you won't have a weak foot. Always work both feet. Work the whole foot too, not just the instep or just the inside. The ability to play one- and two-touch with the ball makes you faster tactically. Being able to use any surface of either foot is important to being able to play one- and two-touches. It always amazes me the number of defenders that will give you the first left foot shot free. Finally, one observation about about playing in general. I can quickly tell how good a player is by watching what the player does off the ball. Offense or defense, most of your time is spent off the ball. Good off the ball play requires an understanding of the use of space. How to defend space and how to create and exploit space. Hope that helps.