Why?

As a general rule you can only create spells or enchant armour with spells you already know and can cast. I think, if I remember rightly, you need to be at least an Expert in the illusion spell and have a a lot of magicka to cast that and the Invisibilty spell. I can`t remeber who sells those spells but it was definately someone in the Arcane University, in the Praxo or Chironasium.
However, can I just say that if you haven`t permenantly closed those oblivion gates yet (by completing the main quest), it may be an idea to obtain four sigil stones that contain the chameleon 30% ability (the percentage is level based, to get 30 per cent you need to be almost at level twenty). The abilities that the sigil stones contain aren`t pre-determined so you can save then reload if neccesary. Once you have obtained four of them enchant them into your armour and the magnitude will be over 100% (this is assuming the magnatide is 30% and not 25 or lower in which case you`ll need more than four stones). This makes you completely dissappear from view, even the guards will not be able to see you. This makes missions requiring stealth a doddle!
Please note that the guards can still aprehend you if you steal from or murder someone that`s within their range but what`s interesting is that if you resist arrest they`ll suddenly lose sight of you.

You need to know how to cast it first.

Otay you have to have the spell(a camelion is the best) then get 10 grand filled with grand souls and the cameilion spell (easyest is the dark brotherhood cham spell because only need 25 illusion) and 5 pecies of armor and 10k gold will be good, enchant and put armor on and you will have invisabily all the time you can do anything you want