'I wanna' is a representation of informal speech. It's often used in song lyrics, or written forms of informal conversation, and emails/social media. It can mean either "I want to" of "I want a". So it means a little bit more than just "I want". For example; I want a drink, so I want to go to the bar. / I wanna drink, so I wanna go to the bar.

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1. This is what 'wanna' is: It is how the verb 'want' and the preposition 'to' (or sometimes 'want a') SOUND when they occur together natural, relaxed speech. For example, when we say "I want to go home" in a careful way, 'want to' sounds like 'wanta'. But we say it fast or in a less careful way, we often drop the 't' sounds, so that it sounds like we are saying 'wanna'( or 'wonna' or 'wunna', depending on the accent). Note that this doesn't happen with the third person: "He wants to go home" always keeps the 's'. Nobody would ever say "He wanna..". The written transcription of this reduced form in is often used in song lyrics and cartoon captions, as an imitation of speech. In recent years, it has also spread to subtitles and even to informal messaging. 2. This is what 'wanna' is not: - It isn't an alternative to 'want'. It isn't even an alternative to 'want to'. It's just an approximate transcription of a sequence of sounds that occur in natural speech. - It isn't slang. Slang is a deliberate choice of vocabulary to suit an informal context, like deciding to say 'buck' instead of 'dollar'. No native speaker consciously chooses to say 'I wanna go' as opposed to 'I want to go' : this reduction/elision is simply what happens when we're speaking. -It isn't especially informal. Even in formal situations, rapid speech will make 'want to' sound like 'wanna'. 3. Should you use 'wanna'? In spoken English? Yes. As your spoken English improves, and you acquire natural intonation, this reduction will happen naturally. In written English? No. Please don't believe anyone who tells you that 'want to' is "proper" English for formal writing, while "wanna' is informal English that we use with friends. This is not true at all. And please, please, don't think it will make your writing look cool or native. Scroll through the pages of italki and you'll see that it's only learners who write 'wanna' - not native speakers. https://www.italki.com/discussion/227739