Don't forget to put a verb after the "going to", otherwise you can't pronounce it as /gonna/. Many pronounce it as /goin tuh/, too.
Subject + be + going to + verb (+ noun)
He's going to cook (dinner).
If you put a noun (place) after the "going to", you can only pronounce it as /going to/ or /going tuh/.
Subject + be + going to + noun
He's going to school.
Remember that /gonna/ and /goin tuh/ are only supposed to be used in speech. It's fairly common for people to use them, along with other reduced pronunciations, in writing to save on time and energy. My advice is to please make an effort to use the entire term-no sense in developing bad habits while learning a language. (Ok, you may use them when sending SMS!)