"I'll get dinner" has two possible meanings depending on context: 1) The speaker will go out and buy dinner and bring it back to eat together. - Do you fancy a takeaway? - Sure what about Mexican? - Good idea let's get fajitas! - Great, let me finish my homework and when I'm done, I'll go and get dinner then - Perfect 2) The speaker will pay for dinner if the other person has paid for breakfast or lunch: - Shall we get the bill? - Sure (Bill comes) - It's OK, this one is on me (person will pay in full for lunch) - Ah OK thank you, in that case I'll get dinner for us later Or: - I only have cash, how about I pay for lunch and you get the dinner? - Sure

So you can say: I'll buy dinner; I'll order us dinner

Or, I'll make dinner.