This is my first guide, so the text will probably be a bit thin, but my item build is set together by 2200 rated players.
Nautilus is a awesome damage dealer while he is so tanky. Feel free too comment and give me tips.

Pros:
+ Crazy CC
+ Insane ganks if your team is in
+ Safe jungle (might be a bit of a pain in the start, but after you get a hold on it, he is quite easy to handle)
+ He is HUGE
+ Insane damage if the enemy team ignores him
+ Ulti will knock everyone up if you target the champion the furthest away in a team fight
+ HE CAN FLY!

Cons:
+ He is slow (really slow)
+ Cooldowns, ugh..
+ His anchor is waighing him down

Dont know if i got the skill sequence right, but you want to max by lvl 9, by lvl 13 and at lvl 18
Train at lvl 6, 11 and 16

I choose to start with and 5x because Nautilus is quite squishy in the start. makes ganking so much easier, as Nautilus got a quite slow base movement.

You can replace with if you are having problems with mana. The reason i choose Rylai's is the slow, it helps so much when your or is on cooldown, and might give the possibility to get in a extra stun.

Gives that extra bit of damage a AP character needs
Better survival against physical attacks
Gives you what this build is lacking, magic resist. (I prefer these over the flat ones ad you dont need much MR in lower levels)
I choose the GP10 because i like to give the kills to the carry, mid or top laner.

Best summoner spell IMO because of the great escape options you get with it, or just to get that last hit to kill a enemy.
This one is quite obvious as you are a jungler. Also you get 10 gold every time you use it thanks to masteries.

U should keep track on the spawn timers, it will help yourself and your team ALOT.

Small minion camps spawn at 1:40 and re-spawn 1 minute after the last creature of the camp is killed.
Red, Blue buff: 5 mins. Spawns at 1:55
Dragon: 6 mins. Spawns at 2.30
Baron: 7 mins. Spawns at 15.00

Well thats it, i will probably add more later, but for now this is it.

Good luck jungling!