It depends on the level of formality. In reality, all they really want to know is how many people are attending so that they know how much food to order. Your reply is probably going to be read by someone in charge of organizing the event, not the person who invited you. Therefore, the most important thing you can do is to make it perfectly clear, at a glance, whether you are coming or not. If the invitation suggests that you can bring a spouse or friend, then make sure you say how many people are coming. The only serious mistake you could make would be leaving them wondering: "I can't tell whether he's coming or not." I would suggest one simple change. Use the contraction "I'll" in place of "I will:" "I'll be there." That is perfectly clear but makes it slightly less formal. If you think it is appropriate to sound enthusiastic, you can add something else. You could use an exclamation point, "I'll be there!" and add another sentence or two, like "I am looking forward to it." If it is an extremely formal situation, you could say "This is to confirm my attendance." You can echo phrases used in the invitation. For example, if the invitation says "please confirm your attendance" then say "this is to confirm my attendance," etc. By the way, in the case where you are not attending, in the United States the customary way of saying you are not coming is to use the specific word "regrets." "Regrets, I cannot attend."

Informally, "I'll be going". A little more formal ... "I'll be attending".

I will attend it or I will be present on that day/event.

You could say something like "See you then!" or "Looking forward to it!" or "Can't wait!" to show enthusiasm while implying that you'll be there.

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