You play passive until you are level 6. Then you have your ulti and you can play agressive. You need to farm, because you need the Hextech for your agressive-gameplay.
If you play against a tank, you max better your q and harass him only.

Your combo:

1. jump to the enemy with your q-spell
2. do an auto-atk(Lich bane)
3. if he tries to escape, activate your e-spell
3. else activate your shield with your w-spell
4. do an auto-atk again
5. do your q spell
6. and do an auto-atk again
... so now use your q- and your w- spells while the mele fight and save your e- and r-spell to catch him if he tries to escape