Runes: attack damage, magic resist, armor penetration. armor pen quint.
Summoner Spells: exhaust, ignite.
Start off the game with doran's sword and a health pot. put a point on your Q and pick your lane. Simply last hit minions. dont stand directly in the thick of the minions. this makes you an easy target for enemy champions. stay smart, and weave in to get your last hits and then out to keep yourself alive. REMEMBER THAT IF YOU DON'T HIT AN ENEMY ON YOUR DASH IN, YOU WONT BE ABLE TO DASH OUT. Playing EXTREMELY defensive early game will benefit you greatly later on. at level two, upgrade E. you can now use your E to dash in towards enemy champions and then immediately out again (it is preferable to dash THROUGH minion waves) and then use your Q to finish off minions and gain some health. This will put your opponents is a very defensive position.
Once you reach level 3, put a point in your W. then set up a gank with your laning partner. hid in a bush and wait for an enemy to get too close to your partner then dash in with E, use exhaust, stun with W and immediately hit with Q. Continue hitting them until they get too close to the turret and Hit them with ignite. this should finish them off, giving you a solid early kill.
At this point you should have a decent amount of money. If you decide to go back, buy the boots in the "recommended items" list and a potion.
Early game kills are crucial to building up ren. Build your Q and W and continue harassing and ganking until you get your ulti.
Once you hit level 6 or 7, look for a gank in the mid lane. If it's an easy kill, take it. otherwise stay in your lane and help push by dashing in and out through minion waves with your E and finishing them off with Q.
You should not focus on leveling your dash until you have to. Focusing on Q and W are crucial. Doesnt matter too much which one. W is better for ganking and spike damage, Q is better for farming.
Your next buy should be the frost mallet (located under "attack" > "damage"). this will make it even easier to kill enemies since they will not be able to run away from you.
Push and gank when you see an easy kill. Get money until you can get warmogs armor. by this point, you are nearly indestructible. when attacking turrets, use your W as much as possible.
There really isnt much more to ren. Keep in mind that your ulti can give you extra health and do AOE damage. So use it in a pinch, or just to help you farm minions. E can be used to escape conflict, and Q can be used to heal. Keep and eye on your fury meter. GO WIN!