please tell me when we can use 'wanna' word?

We use ' wanna ' just like we use ' want a / an / to ' . It is just an informal way of how we adress ' want a / an / to ' . Examples - I wanna go swimming.

There are many such words like lemme short form of let me, gonno for going to, gimme for give me, Gotto for go to...just informal way of saying to make it short.