All nouns have gender in Spanish.  That gender has nothing to do with sexual gender, and so there is no logical reason for the gender of any particular word, unless the noun refers to a human or animal that is gendered.  A good example of this is the word "persona," which is always feminine, even if it is referring to a man.  Articles of clothing or anatomical features associated with one gender or another have arbitrary genders unrelated to the gender of the person who possesses them.  

Rules for distinguishing gender are based largely on spelling.  Words that end in O are masculine (except la mano) and words that end in A are feminine (except a large group of Greek words like programa, problema, tema, teorema, poema, sistema, planeta, faceta, mapa, pesticida) and the word "el sofá."  Further rules for this are explained in the beginning of every basic textbook of Spanish.  

It's important to know that the word for noun in Spanish is "sustantivo."  Therefore, when you are using a dictionary, the italics s indicates a noun.  In English, we use an italics n to indicate nouns.  Some dictionaries do use n to indicate nouns in Spanish.  It varies.

Other dictionary abbreviations include:

adv adverbio--modifies verbs, adjectives and other adverbs.

adj adjetivo -- modifies nouns

tr transitivo -- a verb that has an object (direct or indirect).

itr intransitivo -- verb that only has a subject, or a subject and a predicate.  It does not have a direct or indirect object, but it MAY have a reflexive object (the agent of the verb does action to him/herself.)

m, f --masculine & feminine gender.  There are also neuter pronouns in Spanish.  When the noun you are describing cannot have a gender, or the gender is unknown because the identity of the noun is unknown, use neuter pronouns when possible or masculine pronouns and adjective forms.

When using a dictionary to look up words, remember these rules:

Do not look up conjugated verbs.  Convert the verb to its infinitive, and then look that up.  For instance, do not look up "went" or "seen", but rather "(to) go" and "(to) see". 

Some dictionaries are organized by infinitive verbs, so you won't be able to look up past participles without converting them back to their infinitive.  For instance, the Langenscheidt dictionary is set up so that you don't look up "cansado", but rather "cansar", and then the extensive entry includes the noun and adjective forms derived from that verb root.  Check this out immediately by seeing if you can find these words:   aburrido, vuelto, hablado, querido.  For this reason, I believe that the Langenscheidt dictionary is excellent for students in the second year, but not so good for first year students.

Some dictionaries still use the traditional Spanish alphabet to sequence the entries, so a word including a ch will appear after cz.  For instance, the word cremación appears before the word charlar.  Check this out right away when you buy a dictionary to see which way it's organized. 

Most dictionaries include some clues as to the context in which a specific word is used.  For instance, the word "knot" can be something tied in a string, or a nautical term used to measure speed.  Be sure to read the entire entry to see if you have the right word. 

If you are looking up a word from English, and have a possible translation in Spanish, look that word up on the Spanish-to-English side to verify what meaning that word really carries.  It might give you further information to tell you if it's the right choice.