First of all nunu is a great counter-jungler, that means you should start with wolfs and ask for a smiteless blue. After that head to enemy red part of the jungle and steal the big wraith with your Q. After that head to his red and steal it. After this you have 4 possibilities:
1. You can either wait for enemy jungler to kill him at his red or at least burn his flash (NOTICE THAT THEY KNOW YOU STOLE RED BECAUSE YOU TOOK BIG WRAITH!). That means that high elo wont even check the red since they know its gone (Tip: you can leave them 1 little red golem to prevent them from re-spwaning). Enemy junglers in most cases come to red with <50% HP (not all, e.x WW, Udyr, ...) so you shouldn't have problems killing them. Since you have smite/heal on Q you should be at full HP waiting for enemy to arrive.
2. You can gank top (in most cases you want to do that). Or do that after step 1.
3. You can gank mid if it overextends hard.
4. Continue your jungle path.
Thats about all you need to know about nunu jungle. It doesnt cover details but only the important thing on how to start. Ganking isnt really a mistery since you run with your buff in, slow them and proc your auto attacks that apply red buff to the enemy. If you lost any HP in a gank feel free to heal on a minion before you leave the lane.