Going to is used to talk about intentions or plans that were made before speaking, such as "Jo has won the lottery. He says he's going to buy a Porsche." It is also used to make predictions about the future based on present evidence, like "The sky is very black. It's going to snow." The structure of going to is subject + be + going + infinitive. Going to expresses either something we intend to do based on a prior decision, or a prediction based on current observable evidence.