will

We use will to talk about plans decided at the moment of speaking.

• I forgot to phone my mum. I'll do it after dinner. He decides to phone his mum when she is speaking - she didn't have a plan.
• I can't decide what to wear tonight. I know. I'll wear my black dress.
• There's no milk in the fridge. I'll buy some when I go to the shops.

going to

We use going to to talk about plans decided before the moment of speaking.

• I'm going to phone my mum after dinner. I told her I'd call at 8 o'clock. He decided to phone his mum before he speaks - he already had a plan.
• I'm going to wear my black dress tonight. I need to pick it up from the cleaners.
• I know there's no milk. I'm going to get some. It's on my shopping list.