How to Position
Mids a little more complicated, so I'm going to break it down into separate categories
Control mages are champions such as ahri, and orianna. These champs position near the back line and use their abilities to force an awkward fight for the enemy team while also positioning in a way that forces the carries to stay out of the fight
These are people like ziggs and xearth. They stand at the very back and excel at sieging towers. They poke down enemy champs before the fight begins and generally makes them pick fights instantly
Assasins are people like fizz and zed who excel at quickly killing a single target. They usually stay on the outskirts of a fight and try to catch a carry out and then escape. They have a very in and out playstyle.