Hi , be aware this is my first guide so might be updated and not so good.
Summoner Spells :
As NuNu I like to take the Summoner Spells Flash and Cleanse. I hate to see someone with exhaust as NuNu because its unnecessary with your Ice Blast and Blood Boil.
Flash is basicly for escaping and chasing , it can save your lives or get you a few kills.
Cleanse is helpful if your chasing and you get stunned , they will get out of range of your Ice Blast and if they have boots , they'll have time to get away even with your blood boil.
Cleanse can also get you out of tight situations early game when your at low hp.

Early Game:

Early game I like to solo a lane for : Extra Money as long as you last hit and allowing your team to have 2 people in the other lanes. Keep harassing them with ice Bast but dont get careless and run out of mana quickly , which can happen early game. At level six let the enemies push a bit and hide in brush. When you have an opportunity Ice Blast the once with higher HP and try to use your uti on them. If you dont kill them , chase them and you can dive if your confident you will make it and dont forget to use Flash! If the Enemy has a Twitch/Evelyn or anyone who can stealth get an oracles elixir! You shouldn't be dying much and do you think they'll expect to be hiding in brush waiting for you to push and gank , then realise there in range of your ult , they wont. It wil be to late for them to get out of it so there almost dead now. then just finish them off.

Mid Game:

Ok now you should have your Warmogs and Rylai's. Now you shouldn't be scared to Turret Dive aslong as you keep your HP up with consume and fighting enemies near their minion waves is EXTREMELY helpful because you'll have a nice heal with you.

Late Game :
Just keep killing people and soon you should be able to demolish the enemies and get atleast 14 kills a game. Please Leave Comments and other Helpful things to help players.