First there is one skull in when you have to shutdown the rings there is a building where u fly in the hornet in the inside of the building is a skull.
why?
go on youtube and type in search halo3 skull and it will tell you where all the skulls are.
Normal, heroic, or legendary.
Finish campaign and get all the skulls and do the score challenges and you should have close to 49
to get armor you must get achievement's and to get the haybusa armor you must go through the campain and get all the skulls from level 1 to the end of the game
About 13
By going in spots u never been.Some is easy to to fine and some is not in the last level when u start turn keep moving back and u will see a tunnel or more maybe go into every one and u will find it.
there are none :( to bad don't waste your time looking for them but look for skulls on levels they give you one shot kills unlimited ammo and much much more
lol no no no no no no no no and, guess what? no! (hope this isn't offensive)
youtube...
You don't this was meant to be gave when you find all 13 skulls.
http://www.youtube.com/watch?v=XIE7I2OPgyA guide to all 13 skulls