Don't play champions only because they are high in a tier list, play your main champion even if they are not top tier. I personally would always pick Diana over Elise (I'm not bad with her, just not as good as I am with Diana).
Tipp:
-Farm, don't try to gank all the time.
-Focus on one side (I usually choose to stay between bot and mid)
-don't gank a lane if the enemy laner is fed AF.
-don't fear ganking botlane
-know your champ (powerspikes, item builds, ...)
-mute everybody who can't accept that you won't gank his Lane or flames you because he gets ganked by the enemy jungler. YOU should decide if it's worth to loose gold and pressure to help a lane.