Hey man Im a plat scrub Yi main but maybe I can help you :)
Ganking :
You can definitely gank with Yi pre-6, you are quite dependent of red buff though. You need to hold your alpha, walk to the enemy, apply red buff and hit&run, then if they use a cc/dash/flash or are killable, you Q.
Toplane is often a good bet at lvl 2-3. Not too much after because you need to secure dragons. Ping like crazy when you gank, you need follow up. Lane with heavy CC are your friends. Don't greed if they escape, you should avoid towerdiving pre-6 most of the time. Forget about lanes that are losing hard unless the enemies are way too cocky.
Also : TAKE KILLS. You're not Lee Sin. A Yi fed is always great news. After a kill always push the lane with your laner. Hold the lane when your laner recalls. You should try to always be the highest lvl in the game.
Counterjungling :
Sometimes you won't find safe opportunities to gank, then you focus on counterjungling. If the jungler is at the opposite side of the map, you run into his jungle, ward everywhere/smite wolves and steal as much as you can. At lvl 6 you can 1v1 almost any jungler who's not fed.
Objectives :
People will bitch at you often, playing with a Yi can be frustrating, even my friend is mad at me sometimes but making the hard choices will help your team on the long run.
For runes I use Ascéndant's set (EUW challenger) : red AS, yellow hp/armor or hp/armor/lvl, blue CDR + CDR/lvl, quints AS. You can use whatever you like but you should preferably have 19% attack speed to clear the jungle effectively. I see a someone told you about the farming jungle route (skipping buffs), it's fine but you won't be able to gank early... you should adapt your route anyway, I always get a pink lvl 1 and start at my buffs against an heavy counterjungler just to be safe.