You should preferrably go for short trades with your Grasp up.
Last hit with your Q to get graves. (The moment you hit level 2 you should have 3 graves)
When you have 3 or 4 graves try to hit your E, if you don´t hit it don´t reactivade your Q, because the ghouls will be pushing the wave, but you should never push for no reason.
But when you hit the E reactivate Q and then take one Corrupting Potion to get the burneffect on the enemy using your ghouls. Also your ghouls will stack Grasp, so you can walk towards your enemy and hit him with your Q.
Your true 1 vs 1 potential comes trough your ult, but it is important to hit your E, so your Maiden is going to focus the enemy, so you get the bonusdamage on your Q.
How to use the W should be obvious, but you can also use it the prevent the minionwave from crashing into your tower.
With Yorick you basically don´t want to teamfight, you want your team to draw the enemies attention and to occupy them long enough for you to take towers and inhibs.
Always try the be on the opposite side of your team, so the enemies have a longer way to go to stop you.