Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Buy all the runes that gives maximum ability power at level 1. It makes you seriously overpowered at the beginning. It is critical for you to play as hardcore as you can in first minutes of the match.
At level 1 take Shadow Walk to scout an enemy and stun for your first blood. Then Try to maximize your Ravage and take Malice and Spite everytime you can. This one helps you while fighting few opponents at the same time and also saves your time as you don't need to go recover to base after killing an opponent. You won't need more than one level Hate Spike as it has only slight difference.
Try to get First Blood by getting in lane with someone who has stun. Let him attack first and after his stun ends join in the fight also stunning opponent with your shadow walk. When he starts running like a little girl - use Ignite. That's it - he's dead!
Check on middle teammates opponent - if he has half HP you can try to go help and get another kill or assist. Also, you can check how it is going for your friends in other lane and use Teleport if their opponents has not that much HP.
Keep tracking all your enemies health and try to assassinate everyone you can. It's essential for you and your team to be fed at the beginning. You should be the one who spends more time changing lanes for an easy kill than farming monsters.
Try not to be involved in team-fights. Search for players wondering alone and use Teleport to get to them as fast as possible. If there are no such players go help your team by finishing opponents, but do not attack while enemy is with full HP. Go follow those foes who thinks to get safely in the bushes and Recall back to base. Surprise them with your Ravage spell and then hide in the shadows again...