There are three genders in Croatian: masculine, feminine and neuter. If you don't know the gender of a word, you can not make a grammatically correct sentence.
Luckily the gender of most words is (almost always) obvious from the ending.
Masculine words normally end in a consonant.
Also, if you are talking about a male person or animal, then the noun gender will be masculine. The same holds for feminine nouns.
Examples of masculine Croatian nouns: pas (dog), krumpir (potato), čovjek (man).
Feminine words normally end in a.
Examples of feminine Croatian nouns: žena (woman), karta (ticket), żaba (frog).
Neuter words normally end in a e or o
Luckily, there are not many exceptions to the above rules but ...
Words ending in ost are feminine.
There are also some common exceptions.
The followng common nouns, which look masculine are in fact feminine:
novine (newspaper pl)
hlače (trousers pl)
These have different endings in the genitive case.
There are two words for two dva and dvije.
Dvije is used with feminine nouns and dva with masculine and neuter.