Teleport is your friend, and is the best way to make an impact on the game.
Watch for team fights, especially around Dragon or Baron. Granted this does require that your team wards more than one or two spots, but that should always be happening. As teamfights break out, take a split second to analyze the situation. Ask yourself things like;
Is one or more members of the enemy team out of position?
Is anyone low on health and trying to escape in the path of a ward?
Is my team in an advantageous position or are they currently fleeing for their lives?
(Assuming it's still up, and assuming the game is equal in terms of CS and towers taken) Will the enemy top laner be able to take my tower?
The other means of impacting the game depends on the enemy top laner. If they've got their nose to the ground and are just mindlessly pushing top, that can make it difficult to get out of lane to help the team.
On the other hand, since you like to play Nasus then really you should just be farming the hell out of your Q in lane. Teleport then when you can instantly slow a target and drop your AoE to create an unhealthy zone for the enemy team to fight in.
Can't say much about Renekton, but (as someone who picked him up as their first champ) Garen is a bit of a niche pick for top. He doesn't really bring anything to the team besides being impossible to get out of lane. He's okay to pick if your team already has decent CC going for it, but he gets countered pretty easily by any champ with ranged harass. Champions like Gnar are great because they bring damage and tankiness, but also good CC to peel for teammates or catch enemy champs.
So I'd suggest widening your champ pool a bit and always keeping an eye out for places to make a teleport play.