Eres bebe leche?
Why "Eres bebe leche" is wrong? What's the difference of eres/usted/tú ?
"Eres" means "you are", and "bebe" is the third person form. The correct answer would be "Tu bebes leche" or simply "Bebes leche".
I assume that you're thinking in English and are attempting to translate the phrase "You are drinking milk" straight across to Spanish. Tomparryjones is right that the way that that would be phrased in Spanish is "Tú bebes leche", but if you wish to form a version of that with a gerund (the noun with the 'ing' in english), you could phrase it as "Tú estás bebiendo leche". Tip: while we take liberties with the progressive tense in English, it is reserved for actions that are in progress in Spanish.