If it's before the meal and when you start eating, you can say "いただきます(Itadaki masu.)" or
if you want to tell someone that you are going to eat somewhere from now(or maybe today, tonight...),
you can say "（食べに行きます。）Tabe ni iki masu."
but the second phrase, you have to put whether "where" or "when" or "what" are you going to eat,
or it doesn't make sense.