"The girl is drinking my milk."
Translation:Девочка пьёт моё молоко.
There are a few exceptions, but for most words, if it ends in a consonant or й, it's masculine; а or я, feminine; and о or е, neuter. If it ends in ь it could be masculine or feminine. http://www.russianlessons.net/grammar/nouns_gender.php
Девушка is a girl from the age of puberty to the womanhood. The other meaning is girlfriend indeed, but keep in mind that this can not be used to talk about dating someone who is in her "forties" (which can be easily done in English) - this will sound slightly off.
Девочка is just a child girl. Sometimes woman recognize themselves as девочки ("ну мы же девочки, нам можно плакать") but this is nothing but a figure of speech.
Yes, it's the most confusing thing. For exemple дорого can have two meanings and prononciation depending on where the stressed syllable is: one means "road" (daroga) and the other means "expensive" (dorogo/a). You can get away with a lot of approximation when speaking, but it's very confusing when writing.