"I'm going to eat dinner" is what you say if you intend to eat. "I'm going to dinner" should probably be qualified to explain where you are going, i.e. "I'm going out (to a restaurant) for dinner" or "I'm going home for dinner"

I'm going to eat dinner

Thank you very much everyone for answering my question.

"I'm going to eat dinner." Here, "going to" means "will". This is a special use of "going to". "I'm going to dinner." Here, "going" is just the present continuous form of "go". It means that you're going to wherever dinner is located (probably a restaurant, but it can mean you're just going to the dining room).

"I'm going to dinner" is nearly the same as "I'm going OUT to dinner"