From what I learned today, the gender rules in Serbian seems quite regular. The rule is:

1. masculine nouns usually end with a consonant,
2. feminine nouns with a final -a,
3. neuter nouns with final -o or -e.

It would be nice if the rule is very strict, but there are some exceptions. For instance the word for car auto ends with -o ending, but is considered a masculine and the word for table sto also is a masculine noun.

Possessive adjectives (Prisvojne zamenice)

So the following is a massive table on how possessives are described in Serbian…

Personal pronoun	Masc. Sg.	Fem. Sg.	Neu. Sn.	Masc. Pl.	Fem. Pl.
Ja	moj	moja	moje	moji	moje
Ti	tvoj	tvoja	tvoje	tvoji	tvoje
On	njegov	njegova	njegovo	njegovi	njegove
Ona	njen	njena	njeno	njeni	njene
Mi	naš	naša	naše	naši	naše
Vi	vaš	vaša	vaše	vaši	vaše
Oni	njihov	njihova	njihovo	njihovi	njihove

So if someone points at my car and asks Čiji je ovo? I may reply as Ovo je njegov auto I suppose. Here’s one interesting thing of Serbian though. The word for whose also changes according to the gender of the object of interest. For masculine nouns like sto, it would be čiji, but for neuter nouns like dete, a child, one would ask čije je ovo and for feminine nouns like kuća, a house, the word čiji should change to čija.