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

​Oops, I forgot to phone Mum! I'll do it after dinner.
I can't decide what to wear tonight. I know! I'll wear my green shirt.
There's no milk. I'll buy some when I go to the shops.

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

I'm going to phone Mum after dinner. I told her I'd call at 8 o'clock.
I'm going to wear my black dress tonight. 
I'm going to go to the supermarket after work. What do we need? 

We usually use the present continuous when the plan is an arrangement – already confirmed with at least one other person and we know the time and place.

I'm meeting Jane at 8 o'clock on Saturday. 
We're having a party next Saturday. Would you like to come?
Are you doing anything interesting this weekend?