Nunu early game isn't the best but it still works. You won't be getting as much kills until about mid game. Just try to gank the lanes by providing slows with your snowball and smite. Also blood boil the person who you are with to make it easier for you to take out the opponent. Your main purpose in this part of the game is to help your lanes like other junglers, but also to take the jungles of the enemies as much as you can to prevent them from coming back into the game to some extent.

You have reached mid game and now you are at your power spike. This is when you actually do damage. In this, try initiating by blood boiling yourself and snowballing them (E) to deal dmg. Try to also stay with your teammates to secure the kill just in case they try to run away. Also you can continue on taking the enemies jungle as much as you can if the situation is safe. (But don't let your team die for the jungle)

Late game for you is so powerful. The E cooldown is so low and so is your blood boil. If anything happens, just blood boil, E, and continue on doing that until they die. Try playing near minions or anything you can use your q on to heal because your ap gives you more healing from the bite.

Nunu is a champion which seems simple enough but you have to understand when to go in and when to go out to be able to actually do work on this champion.