How do I know what gender a word is?

In English we don't have to worry about the gender of words, so the idea of word gender is understandably confusing for English-speakers. In Spanish a word will always be either masculine (e.g. el timón/un timón) or feminine (la ciudad/una ciudad). There are some rules to help you decide on the gender of each word.

Feminine
Some of the common endings that suggest a noun is feminine are:

• Words ending in -a

la carera, la enfermera, la casa

• Words ending in -ad

la ciudad, la libertad, la amistad

• Words ending in -ión

la estación, la ilusión, la religión

• Words ending in -z

la nariz, la paz, la luz


Masculine

Some of the common endings that suggest a noun is masculine are:

• Words ending in -o

el teatro, el dormitorio, el cartero

• Words ending in -e

el perfume, el maquillaje, el estante

• Words ending in -ma, -pa, -ta

el programa, el mapa, el planteta

However... There are of course exceptions. Some exceptions are logical. For example the word fotografía is feminine (ending in -a), although we can shorten it in Spanish to foto (like we shorten photograph to photo in English). Even though foto ends with an -o it is still feminine as it is just a shorter version of a feminine word. Other words following this pattern are: la disco, la moto, la tele etc.
There are a small collection of words that you will come across as you acquire more vocabulary that don't follow the rules or have a logical explanation. These are words that you will end up commiting to memory if you use them enough. A good source of information for words breaking the standardized gender rules is:http://spanish.about.com/cs/grammar/a/genderreversal.htm