In Arabic, all words are one of two genders: masculine, or feminine. There is no “it”, no third, gender-neutral gender like English has.
The main thing to remember is that when you don’t know the gender, the default is the masculine gender. You also use the masculine gender for mixed groups (eg. if you’re talking to a group of men and women).
The main sign of a word being feminine (remember, if you don’t know, the default is masculine) is a specific form of the letter ta, called ta-marbuta (التَّأ المَربُوطَة), or “tied-up ta”, which looks like this:
ة
Ta-marbuta also changes some words from masculine to feminine. For example, the word tabiyb (طَبِيب) means male doctor. Tabiybah (طَبِيبَة) means female doctor (notice the ta-marbuta at the end?).
Some feminine words don’t have ta-marbuta (like sun (شَمس), so how do you know they’re feminine? There are only a few words like that, so don’t worry too much about it.
Wallahu ‘alim. Here is a list of some common nouns, both masculine and feminine.
kitaab (كِتَاب): book
baab (بَاب): door
jamal (جَمَل): camel
qalam (قَلَم): pen
kursiy (كُرسِى): chair
qidr (قِدر): cooking pot
baTTah (بَطَّة): duck
sa’ah (سَاغَة): clock/watch
sayyaarah (سَيَّارَة): car
mil’aqah (مِلغَقَة): spoon
Finally, when refering to nouns, since there’s no it, you use the masculine pronoun, huwa (هَوِ) for masculine objects, or the feminine pronoun, hiya (هِيِ) for feminine pronouns. So after talking about a duck (بَطَّة) instead of saying “it is big” you say (literally) “she is big” (هِيِ كَبِيرَةُ) or “hiya kabiyratun”.
(P.S. if you can’t read the Arabic easily, increase your font size, or cut-and-paste into Notepad or Office and change the font size to something large, such as 72.)