Both expressions are basically the same but there are some nuances with each one: "Going to" is more used for events that are more certain to happen in time, whereas "will" expresses and emphasizes merely the intention more. "I'm going to buy a car." "I will buy a car." The first sentence would fit more in a situation where you plan on buying it maybe in the next instance or at a certain day in the week. The second sentence shouldnt be used for that situation but rather to express and emphasize your future, even general intention or will of purchasing a car. Cheers!

There is plenty of information and advice about this difference, so your best option is to search, using these keywords: difference will going to In the simplest possible way, I'd say that "will" is a hypothetical situation, and "be going to" is a prediction based on the present situation. You'll find writers talk about "evidence in the present" when discussing "be going to". There are about five other future forms, so make sure you clearly understand the difference between these first two! :)