There are some easy rules to follow when learning whether to use le or la with French nouns. It isn't exhaustive, but it will save you a lot of work!
In French, a noun is always* either masculine or feminine, singular or plural. But as this isn't the case with the English language, we can find it a long and difficult battle learning when to use which gender. It is, however, important to the French language, so it pays to be as accurate as you can. But don't worry, this cheat's guide will put you way ahead of the game!
It is possible to bulk learn the gender of many nouns, because the word ending can generally indicate the gender (with one or two exceptions, of course!). The following are ones you can pretty much count on in 90% of cases:
Masculine nouns: (le, un, du, il)
-ment (le gouvernement, le monument, le médicament...)
-eau (le bureau, le couteau, le carreau... But watch out! L'eau (water) on its own, is feminine, as is la peau!)
-phone/-scope (un téléphone, un interphone, un microscope)
-isme (le socialisme, le réalisme)
-ier (le papier, le panier, le collier)
-al (le journal, le rival)
-et (le cabinet, le bonnet)
-acle (le spectacle but not la débâcle)
-age (le montage, le village, le massage but not la plage or la page)
Feminine nouns: (la, une, de la, elle)
-tion (la situation, la solution, la station)
-sion (la décision, la télévision)
-ette (la bicyclette, la trompette, la cigarette but not le squelette!)
-ance (la connaissance, la dépendance, la ressemblance)
-ence (la différence, la référence, la présence, une agence but not le silence!)
-ure (la culture, la peinture, la nourriture but not le mercure, le cyanure)
-ode/ade/ude (la méthode, la salade, la certitude, la solitude, une attitude)
-oire (la gloire, la victoire, la mémoire but not le mémoire - see below)
-ique (la critique, la musique, la boutique)
Nouns that change gender according to whom you are referring.
Some nouns specifically refer to a man or woman in a certain role, such as une tante or un oncle, so we can't confuse the gender. In other cases, it is a matter of using a feminine form for a woman or girl and a masculine form for a man or boy, so it is usually fairly easy to get these right, too...
un ami, une amie
un client, une cliente
un voisin, une voisine
un étudiant, une étudiante
un Français, une Française
....and so on.
Similarly, some professions also change form according to whether you are talking about a man or a woman.
un musicien, une musicienne
un vendeur, une vendeuse
un acteur, une actrice
...but you need to be aware that some nouns do not take a feminine form, even when you are talking about a woman or girl. These are some everyday ones that you should commit to memory, although just when you think you're getting the hang of it, the French have started inventing a feminine form to nouns which have always been masculine, such as une professeure, une écrivaine, une maire, une ministre, etc...
un agent de police
un docteur, un médecin
Nouns that change meaning according to which gender you use
A number of nouns can take either gender to change their meaning*. As with English, a few nouns can have more than one meaning. Arguably they are each an individual noun. This is clearer in French, where using le or la can indicate the exact meaning of the noun. Because of this, it is worth trying to commit them to memory. Here are some examples:
le tour - tour
le mode - method
le poste - job
le mémoire - report
le livre - book
le voile - veil
la tour - tower
la mode - fashion
la poste - post office
la mémoire - memory
la livre - pound
la voile - sail
* Après-midi can be used with either gender, without changing its meaning! A very few nouns change gender when used in the plural form: amour 'love' and délice 'pleasure' are masculine in the singular and feminine in the plural. Also orgue 'organ' is masculine, but when used in plural form to refer to a church organ it becomes feminine (les grandes orgues).