I am learning also Amir. I believe what you have said is right though (await confirmation from a German!)...except it would be "eine Pizza" as it is feminine. I won't explain Genitive as I do not want to give you false information. I know that 'von' is very often used instead though. So instead of 'Wir mussen den Vater des Jugendliches sehen' you could say 'mir mussen den Vater von dem Jugendliche sehen'. So the 'VATER' is in the Dativ case not the Genitive. It's easier to learn while you get your head around Genitive. :)

The genitive case is correct here. The complete sentence should read: "Eines Tages habe ich eine Pizza gemacht" (note that "Pizza" has to be capitalized and is feminine"). This usage of the gentitive is specific to time expressions. Otherwise it is mostly used in situations where the English language uses the preposition "of" or the possessive suffix "'s". Examples: my father's house --> das Haus meines Vaters the frame of the picture --> der Rahmen des Bildes

I am also just learning. "Eines Tages" is in adverbial genitive case because it's an indefinite time expression (without a preposition). "The time expressions morgens, mittags, abends, nachts, eines Tages (one day) and eines Nachts (one night; analogized with eines Tages, though Nacht is feminine) use the adverbial genitive." Wikipedia