i dont get muramana

It's not something you just buy, it's an auto upgrade from Manamune, when it's fully stacked at +750 mana.

When Manamune transforms and you can toggle Muramana's active effect on and off (essentially like toggling between the 2 items) using the item hotkey.

Check the passive mana charge. Once you cap it out, it transforms.

First thing: Fix the title of your posting to "I don't get how Muramana works"

As to your post, I'm not sure what you mean so I'll try to clarify the item itself.

Starting at tear of the goddess (the first stage): every time you use a spell, you expand your maximum mana you can store, up to 1000. you start at 250, and you need 750 more. The item encourages spamming to "charge" your tear to its maximum. If you charge your tear to 1000 and you buy manamune, it immediately upgrades to muramana.

Manamune: is just a transitional point/item (the second stage) after tear, but before Muramana.

Muramana (3rd stage): if you turn it on, it consumes your mana to give more powerful autos.

Hope this post answers your question! <3

• You use abilities to charge it.
• It can charge it up to +750 mana (full)
• When it's full the item changes it goes to this : http://leagueoflegends.wikia.com/wiki/Muramana
• The only difference : auto attacks and single target spells = bonus damage when item is active
• The down side for this bonus damage = cost 3% of current mana each time.