With Widowmaker, you want to find nice points where you can easily shoot at chokepoints but also watch the objective if you need to. I tend to go for chest shots at full power most of the time since you can easily pop another bodyshot for the kill or leave them for teammates. Full power shots will kill Tracer, Zenyatta, and regular D.Va free so don't both with headshots unless you need to. Don't ignore tanks either, since bodyshots behind Rein's shield or shots towards D.Va's head add up fast.
At close range, just spray and aim for the head like you would with Tracer. I jump a lot to make things harder and if I can grapple I'll do that to make myself harder to hit. For poison mines, put them on random hallways that enemies can use to flank to help keep an eye on who is where. I also like to shoot them on control points to just add some extra damage.
Genji is tricky, you may want to check out Seagull's videos of him playing him. Obviously not everyone is as good as him but his movement show you how to effectively move around and more or less how to use him. I tend to use quick melee a lot with him as at close range you can do alt fire, dash melee, quick melee for a quick combo. Or alt fire, quick melee as a panic string in close range.
His counter is pretty easy to use. I like using it against characters who fire a lot of bullets like Bastion and 76 for easy kills. You can also use it to stop ults like Pharah's and Reaper's to stop them.
His ult is kind of hard to use but generally you want to use it after flanking the enemy team while they're clumped together to break them up.