"Péter is cooking the dinner today."
Translation:Ma Péter főzi a vacsorát.
Those little words (when or where) are among the more freely placed elements of a sentence. Keeping the other three parts of the sentence in the order they're currently in, I'd say that any of the four possible positions for ma is fine:
Ma Péter főzi a vacsorát.
Péter ma főzi a vacsorát.
Péter főzi ma a vacsorát.
Péter főzi a vacsorát ma.
Only the second one has a different emphasis from the rest. The other three seem nearly identical to me - no more difference among them than there is between "Today, Peter is cooking the dinner" and "Peter is cooking the dinner today" in English.