As diamond top laner, your goal should be to win the game.
How you do that depends on the champions in the game, and your elo.
For example, if I'm playing hecarim in bad tier, my goal is to get first blood, roam mid immediately, get a kill easily, then back and teleport into enemy jungle and get a third kill, then just extend my lead by roaming all over the map post 6 where i can just ult my enemy toplaner and kill him because he doesn't know how to handle it.
If I'm playing hecarim in good tier, my goal is to shove lane hard, ward enemy jungle with pink ward and farm/push hard until trinity while teleporting bot if I can get easy kills. After I push out top lane I'll grab tower, harrass enemy jungler and mid lane while taking all the scuttle crabs i can for map vision while building jungle item for HP, then CDR.
2 entirely different plans for entirely different elos. Certain champions in the game will change my plan and how I impact the game to be the most efficient.
If you want to be a really good top laner you have to know how to play any way you have to and know when you have to do certain things. Control Hecarim vs Clippity Cloppity Hecarim is a good example.
But what I want from a top laner is generally just to do all they can to screw over the enemy jungler, midlaner, and enemy top laner, even if it means putting themself behind as well. General rule of thumb:
Top lane -> put enemy behind (annex their homes and burn their children)
Mid lane -> get yourself ahead (roam to get yourself ahead)
Jungle -> get everyone else ahead (roam to get others ahead)
Support -> get everyone else ahead (roam to get others ahead)
ADC -> get yourself ahead (roam after enemy tower gone)
but this changes in the meta and what champion you are playing.