3v3 or 5v5: Build an early tear, get merc treds. finish manamune, Atmogs or trinity after that, players preference.
Dom: Same build, however if you prefer it is sometimes more viable to get Tiamat than manamune. As Tiamat is slightly cheaper and mune is only better at full stacks wich may not happen in shorter dom games.
With regards to his ulti, It's best to use it on an AD carry. Best case they ignore it and it dishes out high damage, worst case they burst it down quickly and are forced to wait for cooldowns.
Furthermore some players have problems with his ult as they view it as a defensive mechanic. It is useful in that regard but can really increase Yorick's DPS as well. I find the best time to use it is not when you fear death, because most players will just book it from your reincarnation since there really is no benefit for them to kill it. Instead use your ulti at the start of or towards the middle of a fight that way it get's hits in as a ghost and catches a lot of players off gaurd.
With regards to yoricks ulti a simple way to think about it is: It's cool to get kills after death, it's cooler when you don't have to die to do it.
EDIT: Also remember that his Q boosts his movement speed, use it on minions to both close distance and escape. Since yorick is kinda tanky sometimes when running from combat it's best to pop q, turn around and hit your pursuers and then flee.