"I eat breakfast."
Translation:Je prends le petit-déjeuner.
We usually don't say "je mange un petit-déjeuner" notably because breakfast is as much about drinking as eating. Similarly to the English "I have breakfast", we say "je prends un/le/mon petit-déjeuner"
In English articles are not as often used as in French. So if you get "I have breakfast", you will need an article or a possessive adjective (see above), depending on context.