Translation:I eat lunch at a restaurant.
From what I've learned, "ni" is more of a generic accusative case signifier, while "de" is specifically for when something is helping or facilitating a certain action.
For instance, "I eat lunch in the park" would use ni, because parks don't generally provide food, it's just the place you choose to eat your lunch. However "I eat lunch in the cafe" would use de, because the de signifies that the cafe provided the lunch you ate.
I could be off the mark with this so I welcome any critique.
Hi, it can mean habitually, regularly, or it can mean something you are doing right now, or about to do - "I am (just) going to eat lunch at a restaurant.. ".
For the "doing right now", you can also say: Resutoran de O-hiru-gohan wo tabete-imasu
This "te-iru" or "te-imasu" form of "doing something now" gets introduced in a different skill.