When you use "I will...", you mean you do something in the future. When you use "I'm going to do something", you mean, that you want to do something and you are preparing to do this

Will is the future simple and it's used to predict the future or to express an immediate decision. Going to is an intentional future so it denotes the intention of diong something. I hope I helped you!

You can simply search for "difference will going to" and you will get hundreds of sites which explain it.

Here's one:  http://www.grammar.cl/Notes/Future_Will_vs_Going.htm