Don't ever be too dogmatic when talking about good or bad English. Although I hate it, and it makes me want to scream "the dictionary is wrong," the fact is that "wanna" is listed as a word in:
Informal
1. Contraction of <em>want to:</em> <em>You wanna go now?</em>
2. Contraction of <em>want a:</em> <em>You wanna slice of pie?</em>
<em>—used for "want to" in informal speech and in representations of such speech</em>''
informal
<em>Want to; want a.</em>
‘you wanna know how low I've stooped?’
‘I wanna guitar’
Worse yet, I have to say that they all are probably right, because <em>wanna</em> has been used in print, in song titles and song lyrics, <em>at least</em> since 1928, the year of the hit song "I Wanna Be Loved By You."
So, I am very sorry, truly, but "wanna" is an informal but legitimate English word. It's as shocking to me as it was in the 1960s when a dictionary first included the word "ain't," but it is a word.
The monumental Oxford English Dictionary established in the 1800s that English dictionaries are based on "historical principles." They record the language as it is actually used, not as English teachers might wish it to be used. If English speakers say something often enough and long enough, it <em>becomes</em> a recognized word.
I can give many reasons why people who are learning English should not try to <em>use</em> "wanna" themselves, but that's a different topic.