At early game, make sure you have a tank with you that can stun enemies, like Leona, or Blitzcrank that can also pull them to you to kill them. Make sure, that when you are about to attack someone, you have double strike ready, so you can "Q" and "E" to them and double strike then ignite if you didn't kill them and flee to safety. Farming is also highly recommended so make sure you "last hit" all the time.

Start not going alone in the jungle especially if you aren't fed. Follow the build i show you step by step, and use the right combo when attacking someone, "E" , "Q" and "R" for a refresh of the skills. Farm more, and remember to push, using "E". If you are caught alone, use ulti to get away or even flash out of the war-zone.
*Note, that the best time to get a penta is a mid-game teamfight.

Try to get a full build and stack your Bloodthirsters. Never, ever, go into the fight first. Let the tank go in and stun them, and when they are focusing someone else other than you, start hitting them, "Q", "E" , "R" , dont worry about your life, your 60% lifesteal of the 1500 crits 2.5 attacks per second will be enough to keep you alive in battle, as lond as you keep attacking. Do NOT stop hitting someone, or your life is gone for sure. Othe than that, it would be easier if you backdoor them , while they're attacking your teamates. You can definately kill them all, autoattack autoattack autoattack autoattack.
Easiest way to get penta, if lucky enough. ;)

They are your best chance of survival, stay behind the tank and never play too aggresively.
Have FUN! :)