"Wanna" is just an informal combination of "want" and "to", said very quickly. You'll hear some english speakers say, "want ta" when talking fast, and skipping the t gives us "wanna".

Robson, please understand - this is a not simply a question of formal and informal English. For example, in a formal essay you write 'does not', and in an informal piece of writing, such as a comment here on italki, you write 'doesn't'. 'Doesn't' is an accepted form of the written language. That's fine. In a formal essay you write 'children' and in an informal situation you write 'kids'. That's fine, too. 'Wanna' is not in the same category of 'informal' as 'doesn't' and 'kids'. 'Wanna' is a transcription of a sound that we make when we are speaking. You are right that you shouldn't write 'wanna' in an formal essay, but you shouldn't write it most informal pieces of writing either. 'Wanna' is a feature of SPOKEN English. It's what 'want to' sounds like when we are speaking in a fast and relaxed manner. It is normal to say 'wanna'. It is not normal to write it, unless you are quoting song lyrics or cartoon dialogue, for example. Or you might use it in a short jokey text message to a friend, e.g. 'Wanna come?'. Take a look through the pages here on italki. You will see that native speakers - American, British, Canadian, Australian - NEVER write 'wanna' in their questions, answers, essays and comments here. Why? Because, as Peachey says, it looks childish, and sometimes even lazy and rude. Only non-natives write 'wanna'.

"I wanna be a hero" also makes you sound like a little child. You won't make mistakes by using "want to", so just stay with normal English and you'll be right.

"wanna" is both words joined together of "want to" it is a very lazy way of pronouncing and writing "want to" for an english speaking person. It's very informal. The correct way is "want to".