"We drink the tea."
Because "the tea" is (a) specific (the "the") and (b) a direct object (it's the thing that gets drunk).
Specific direct objects take the accusative case in Turkish, so you need to use the accusative case form çayı.
Biz çay içeriz with the nominative çay would be used for a non-specific direct object: "We drink tea"; "We drink some tea". But not for "We drink the tea".