Originally Posted by SoLInferno
I should say, Im not looking for "Jayce>Irelia" or something like that, I'm looking for more like "early game aggresion>Vlad"
It'll take some experience to know what to do against certain champion types. My hard and fast rule is, when I can afford to, do what they don't want me to. If they're playing passively and just trying to farm, do your best to get harass in. If they're playing aggressively, play passively - avoid damage and farm as well as you can. If they push the lane and leave, push it back and follow them, or push it and go back.
For example, I play a lot of ren top, and I usually play him really passive pre-6: a lot of the time, the enemy top will get too aggressive and try to dive me, which I can usually shrug off for a kill or at least an even trade.
If they're holding back, I dash in and do my combo and leave. I don't feel pressure to force a hard kill because forcing them to go back over and over if you have the advantage means the game in terms of cs and exp.