"I like milk."
Well... I'm afraid that such a general question is rather difficult to answer.
Here you have:
"lubię" (the pronoun 'ja' is omitted because it's redundant - the 1st person singular form "lubię" makes it obvious what the subject is)
and "mleko" (it looks the same as Nominative, as it's neuter - but it's actually Accusative.
Accusative is probably the most common case, and it's needed by a lot of verbs, including "lubić").