What's the difference in using I'm going and
I will in a sentence
Like I'm going to meet a friend vs. I'll meet a friend.

Question

What's the difference in using I'm going and 
I will in a sentence
Like I'm going to meet a friend vs. I'll meet a friend.

Answer

Using "will" feels more immediate, like it just came to your mind you want to visit your friend.

Using "going to" hints at the fact that you have made plans to visit your friend before.

Have a look at this for more details: http://www.grammar.cl/Notes/Future_Will_vs_Going.htm

Answer

I'm going to do  = a planned action
I will do = a spontaneous decision