Learn Croatian: Kupujemo bocu soka. - We are buying a bottle of juice.

The word soka is in the genitive case, which is used here to mean "of" after a word expressing quantity. Because sok (juice) is a masculine noun, it takes the ending -a in the genitive singular, making it soka (of juice).

Why is the word for bottle bocu instead of boca?

The noun boca (bottle) is the direct object being bought, so it must be in the accusative case. For feminine nouns that end in -a, the accusative ending is -u, which gives us bocu.

How do I know if sok is masculine or neuter, since both get the -a ending in the genitive?

You can tell by looking at its basic dictionary form. The word sok ends in a consonant, which means it is a masculine noun. Neuter nouns usually end in -o or -e in their basic form. Both masculine and neuter nouns share the exact same -a ending in the genitive singular case.

The English translation says "We are buying", but where is the word for "we"?

In Croatian, you usually drop the pronoun mi (we). The ending of the verb kupujemo (the -mo part) already tells you that "we" are the ones doing the buying.