Use be going to _____(base verb) to talk about a future that was planned. Some websites have tests that indicate "be going to" is very different from "will." In real life, the difference is very small. We usually start with "be going to" for a plan and "will" for first volunteering or considering possibilities, but if we keep talking, we may use them interchangeably.
Going to is pronounced like (a cup of mustard) "gonna." Even though lots of people--and many lyric writers-- write "gonna" especially in texts, it's not OK in a formal situation. You should write "going to" and say "gunna."