In what cases do i have to use "піцу" and "піца"
Піца (Nominative case) in case it's the subject of the sentence.
Піцу (Accusative) in case it's a direct object ("receiving" the action, the action is done "to" it)
It's a bit confusing how liking is considered an action.
Story of my life
I don't understand. If both times I'm referring to myself, why is pizza changed?
In the first sentence it's in Accusative, because it's the object of the verb "to love". In the second sentence it's in Nominative because it's the subject of the sentence.