So how do you know what the gender of a noun is?

1) Learn it when you learn the vocab.

When you look up a word, or try to remember it for a test, or use it in a composition, remember its gender too. (Dictionaries always mark the gender, usually with an m or an f, often combined with an n for noun.) Think of the word for 'watch' not as montre, but as la montre. If a noun begins with a vowel or a mute 'h', use the indefinite article. A clock is not horloge, but une horloge.

Une horloge

It's much easier to remember a gender if you learn it from the beginning, along with the word itself. If you have to go back later and try and attach a gender to a word you already know, it will stick about as well as a crêpe to a well-oiled skillet. Or, heaven forbid! if you learn the wrong gender first time round, it becomes a Herculean feat to try and replace it with the correct gender.

2) Learn the genders of common endings.

It's the ending (usually) that determines the gender of a word. La chemise (feminine) is a (men's) shirt. Le chemisier (masculine) is a (women's) blouse.

La chemise

Le chemisier

The difference between them has nothing to do with logic, nothing to do with the number of frills on the cuffs, nothing to do with who will wear the shirt, but everything to do with the -ier ending, which is a masculine ending. And so, thankfully, endings can be used as shortcuts to learning genders.  

The following is a list of masculine endings:

MASCULINE Endings:
Ending	Examples	Exceptions
-eau	le tableau, le gâteau, le château, le couteau	l'eau (f.), la peau
-age	le potage, le fromage, le message, le village	la page, une image, la cage, la plage, la nage, la rage
-isme	le communisme, le capitalisme	--
-ment	le moment, le logement, le bâtiment	la dent, la jument (mare)
-et	le poulet, le buffet, le regret, le beignet (fritter)	la forêt
-oir	le couloir, un espoir, le miroir, le soir	--
-ail	le travail, l'ail (m), le détail (also: le soleil)	--

And here are the feminine endings: