"Cet" is used in front of masculine nouns that begin with vowel sounds. Lait is masculine, but the first letter is a consonant, so you would use "ce" here.

Examples of cet + masculine noun: cet arbre, cet appartement, cet homme. (Homme is interesting because even though it begins with a consonant, the h is silent so the first sound is a vowel.)
Examples of ce + masculine noun: ce stylo, ce garçon, ce chat.
Examples of cette + feminine noun: cette femme, cette pomme, cette boîte.

What was the rest of your answer?

That's correct. Possessive adjectives replace the articles.

Should be vous buvez not vous bouvez

Ça is a pronoun (replaces a noun). Tu aimes ce poisson ? Oui, j'aime ça ! (ça replaces "ce poisson").

Ce is a possessive adjective. Which milk? This milk or "ce" lait. Possessive adjectives are always followed by a noun and they tell whose or which one it is.

ce, cet, cette are possessive adjectives meaning "this" or "that."

cette femme → this/that woman
ce garçon → this/that boy
cet homme → this/that man (note that "cet" is used in front of words beginning in a vowel sound)
ces gens → these/those people

Ça is a pronoun that replaces a noun. Tu aimes la viande ? Non, je déteste ça. (Ça is replacing "la viande").