"I drink milk every night."
を is for direct objects. The topic or subject is/ was/ will be interacting with the object through the verb. In this case drinking the milk.
は Is the topic market. It is very similar to the subject. I'm many sentences it shows who or what the verb is referring to. In this sentence in particular, the topic is omitted and must be assumed from context. Since there is no context, we just assume 'I'. If the topic is 'I' and not omitted then the sentence is: 私は毎ばんぎゅうにゅうをのみます。