Hello welcome to my guide I have been playing nunu since I was born. I am currently ranked in the top two percent of nunu players on euw! I hope my guide can help you! :D
https://www.twitch.tv/swote47 I am usually live here if you want to come by :D

Nunu & Willumpis a tanky jungler who provides a large arsenal of cc (crowd control) and objective control via Q. Nunu also has great gank potential via his W ensuring almost every gank will be successful in most situations.

Pros:
High Crowd control
Attack speed boosts via passive
Great team fighting
Brings surprising damage to team fights
Great clear speed
Fantastic objective control
No problem with jungle sustain

Cons:
Ultimate ability is a channel (takes time to charge up)
Ultimate can be disrupted by any displacement or silence
No built in scaling unlike other tank junglers such as Sejuani and Rammus
If behind you will not be tanky making it hard to get back into the game

your pathing on nunu is pretty simple you can start either blue sentinel > Gromp > Greater Murk Wolf or Red Brambleback > Crimson Raptor > Ancient Krug. Then go into a gank on mid or bot and top prospectively. Once you have ganked it should be around 3:15 and scuttle should be spawning. When taking scuttle you can burst it with a combo of W to break the shield on the scuttle and then proceeding to Q and smite to deal a high burst on the scuttle guaranteed to secure it. After this you can go and clear the side you have not yet cleared and then look for another gank before recalling.