It will not always be possible to predict the gender of a noun by looking at the word. When you look up any Spanish noun in the dictionary, the first thing you'll find will be the letter m (for masculine) or f (for feminine). When you learn a new vocabulary word that is a noun, you must memorize the gender of the word as well as the actual spelling of the word. Sometimes, the spelling of the word itself may make it possible to ascertain the gender of a word just by looking at it.
Basic rules
The first question you probably have is, “When is it possible to know the gender of a noun by looking at the word?” In some cases, you will be able to tell the gender of a word based on the ending of the word itself. The two most basic rules about gender are based on the last letter of the noun: A word that ends in –o is masculine, and a word that ends in –a is feminine. There are a few exceptions to this rule (for example, la mano and el mapa). Check out the following examples: 
Feminine
In addition to the general rule that nouns ending in an –a are feminine, another feminine ending is the letter –d. Typically, a word that ends in –dad, –tad, or –tud will be feminine. Notice that most of these words have English equivalents that end in – ty. When you see a Spanish word that ends in –dad, –tad, or –tud, change the ending to –ty to recognize an obvious cognate.
With reasonable confidence, you can bet that a word ending in any of the following combinations will be feminine: –ie, –umbre, –ión.
English words that end in the suffix – tion are equivalent to the Spanish ending –ción. A Spanish word that ends in –ción will always be feminine, will always have an accent on the ó, and will usually have an English cognate that ends in – tion. For example:
Masculine
In addition to nouns that end in –o being masculine, nouns that end in –or, –és, or – ón are generally going to be masculine as well. Here are a few examples:
Article and suffix changes
Occasionally, a noun's spelling will remain the same but the article will change to indicate whether the meaning is masculine or feminine. Also, sometimes a noun can be changed from masculine to feminine simply by changing the ending from an –o to an –a.
Change the article
When a word that ends in –e is used to refer to a person, the same word is used for both genders. Only the article and adjectives that modify the word will reflect the gender of the person that the noun represents.
It is common to add the suffix –ista to a noun to create a new noun that means “a specialist in …” or “a player of ….” For example, el futbolista is a player of fútbol (soccer), and el pianista is a player of the piano or a specialist in the piano. The unusual thing about words that end in –ista is that they will be used for both males and females. The article will indicate the gender of the person the noun represents. Notice that it will seem strange to use a word ending in –a to refer to a male. If you make a note of the nouns that break these general rules when you learn them, you'll avoid a lot of mistakes later.
There are a few occasions where the exact same word will have two very distinct meanings, one when the noun is used with a feminine article and another when the same noun is used with a masculine article. The spelling of the word does not change—only the gender determines which of the meanings is appropriate.
When referring to people, you can change a word from masculine to feminine simply by changing a noun that ends in –o to –a.
Nouns that end in –or, – és, or – n are generally going to be masculine. If you want to use one of these nouns to refer to a female, you can change it to a feminine form by adding an –a to the end of the word.
Remember that you can only create a feminine version of a noun if the word represents a person. You cannot, for example, change el libro (the book) to la libra just because the book belongs to a girl. The word for book, el libro, is always masculine, no matter whose book it is.
Irregularities
The gender rules, of course, have exceptions, but some are predictable exceptions. Even though a word that ends in –a is usually feminine, many words that end in –ma are actually masculine. Notice the irony that – mawords are masculine. It's even funnier to remember this rule since a word that ends in – dad is usually feminine.
Remember: – ma words are masculine and –dad words are feminine.
Some exceptions are:
Some words may appear to be feminine but are actually masculine. They end in the letter – a but are indeed masculine. Unfortunately, there is no trick to remembering which ones they are, so you just need to memorize them.
A noun that ends in the letter –e may be either masculine or feminine. If a noun ending in – e refers to an object, it is usually a masculine word, but not always. It is safest to memorize the gender of these words by learning them with an article.
There are more than a few feminine nouns that end in – e. Following are some of the most common and easiest to learn:
Some exceptions are easy to understand. For example, the words la fotografia and la motocicleta are typically cut short in Spanish to la moto (the motorcycle) and la foto (the photograph). The original, longer version for each of these words ends in –a and is obviously feminine. However, the gender of the word remains feminine even though the abbreviated version of the word ends in –o.