'wanna' is just the sound of 'want to'. It is very informal, so you should only use it with (say) friends, or when texting. You should never use it in formal writing - always write it as 'want to'. Even using it here on italki is not a good idea, as it sounds like whining.
Never use 'wanna' if you don't need the 'to'. e.g. 'I wanna an icecream' means 'I want to an icecream' which is completely wrong. For some reason language teachers in China seem to think that it is OK, or cool, so teach it to students, whereas it is not common other than as a transliteration of a spoken sound, or being written by ignorant people.