Is this an accusative sentence? I am trying to understand what is nominative vs accusative.
Nominative and accusative forms are often identical. Most neuter nouns and masculine nouns referring to inanimate objects have the same form in the nominative and accusative singular. Feminine nouns ending in a consonant also are the same in both nominative and accusative.
The forms where there's a distinct accusative form is most feminine singular nouns, and in the singular animate masculine nouns (and some inanimate ones, too) have the same form as the genitive.
The accusative and genitive plural is best learned as you go, the main pattern being if the noun is a masculine person, the accusative and genitive plural form is the same. Otherwise, the nominative and accusative are the same in the plural.
When should Czy be used? Is it the the same to say Lubisz ser? and Czy lubisz ser?
"czy" makes it clear that it is a real Yes/No question.
So "Czy [ty/] lubisz ser?" is definitely "Do you like cheese?", while "Lubisz ser?" is most probably "Do you like cheese?" as well, but in theory it could be also a surprised "What?! You like cheese?!"