They're often interchangeable. "Going to" is used to talk about plans that are already made, whereas "will" is used to talk about decisions you make in the moment of speaking, as well as the results of a possible future condition.

be going to, it is sth that u r sure it is going to happen. e.g. it is going to rain later. That means, surely, it is going to rain.. 'will', usually will be used for sth unsure. e.g. I will go abroad some day. That means, u will not quite sure abt this, buy maybe, u will.