"Going to" is used for actions that have been planned. "Will" however is used in situations in which the speaker did not really plan something. For example, if you've been planning on doing something you would use "going to". - I'm going on a trip to China this year. - I'm going to submit the application about which I told you about yesterday. - I'm going to be the best in the class! While, as I said, "will" is used for more unintended actions which we haven't planned. - It's a bit hot in here so I will open the window. - My mom has just finished cooking the pie I like so I will now eat it. I hope this helps you!

In a sense, they are both similar, like in the context 'I am going to cut the grass' or 'I will cut the grass'. The main difference is 'time' I am going to suggests you will do it now, unless you say 'I am going to cut the grass at 6pm'. Whereas 'I will cut the grass' doesn't say when, it could be in an hour, it could be tomorrow etc unless they said 'I will cut the grass now' or 'I will cut the grass later'.