If you want to speak good English, don't use it - ever.

It's just another way to say "going to" and should generally never be used when written, unless you are talking with close friends or attempting to say something comical.

"gonna" is the way most Americans pronounce going to when they are speaking quickly and or informally. You shouldn't ever write it unless your are writing dialogue.

When you want to be ghetto :P

You use "gonna" when you want to sound like a native speaker of (American) English in day-to-day conversation. This is the natural pronunciation of "going to" in everyday speech, particularly in American English. Note that this is not lazy or improper speech. Speakers of ALL dialects of English use connected speech. Please see: http://www.bbc.co.uk/worldservice/learningenglish/grammar/pron/features/connected.shtml For further info, see http://www.bbc.co.uk/worldservice/learningenglish/grammar/learnit/learnitv165.shtml http://english.stackexchange.com/questions/62045/how-often-do-people-say-gotta-wanna-or-gonna-in-english-speaking-countrie