If you're defending try to fight on the point. It's best if your team is behind/around it. They are forced to walk into a bad position.
For payload hold your defenses at strong chokepoints or around corners and don't run ahead to fight them (putting you in the chokepoint / around the corner)
Learn hero counters and make sure you switch mid match.
I think at least half of my matches lost are because people do not hero swap to counter theirs. The other half is just people not cooperating of course.
Do NOT challenge snipers to a long distance 1v1. Duck, cover, and find a different position. (May seem obvious but in game I see it all the time.. People trying to shoot down a sniper from long range and then dying.)
Lastly, watch some competitive matches so you can get a feel for how to play certain maps, heroes, etc.
Oh, and stay away from Genji because you definitely can't play him right.
http://www.youtube.com/watch?v=iLqDNSF_C4o
https://www.youtube.com/watch?v=k9xBuPkcspsany reason for no love for the junkrat in these high level games? I like that guy very much