Jövő idő - will / going to - mikor melyiket?

Egy kis segítség, hogy könnyebb legyen használni a jövő időt angolul.
Angol órák Skypeon: esztigottschal...
-There's no milk
-Alright then, I'll go and buy some.
-There's no milk
-I know. I'm going to buy some when I'm done with my homework.
-I think the Democrats will win the elections this year.
-The Democrats are going to win the elections this year, because they already have most of the votes.
-I'll help you tomorrow if you'd like.
-Don't worry, I'll take you home!
-I'll get it.
-I'm going to go on a holiday next week.