Also, if you want a chance to come from behind, you'll need to understand that some characters have 'expiration' dates as the game progresses. Based on your team comp, you either need the game to be over at the 25-30 min mark, or you need to hold out longer.
Pantheon is a classic example of a char who has a fantastic early/mid game. Early Phage+brutalizer and start roaming. But as the game continues, his viability falls off into just utilizing his W as an initiate. And Support chars are a must. I play Janna, and over the weekend I think I won 8 straight games with her and always coming from behind and down on towers. And it wasn't a 5vs5 premade, just me and 1 other dude.
After you save you're teamates from certain death a couple times, they tend to stick around you and so you can lead the team a bit.