How do you know if a word should be feminine or masculine?
Some words you change the ending. For example, muchos can change to muchas or necesita and necesito. And some words are automatically feminine or masculine like biblioteca. You wouldn't change that one. How do you know when/what words to change or do you just have to memorize which words are masculine/ feminine?
3 Answers
Hello countrylova, I see that you are new here. Welcome to the forum, we're happy that you're here!
Hola amigo, bienvenida a este foro.
One of our rules is that we use correct spelling and grammar. Please keep that in mind in the future. 
Some words you change the ending.
This is true, to some extent. Certain nouns change meaning depending on what gender you use. For example, el policia means police officer and la policia means the police force (or female police officer depending on context).
for example, muchos can change to muchas or necesita and necesito.
Mucho is an adjective. In Spanish adjectives conform to the gender and number of the noun that they modify. This means that if I were to say there are a lot of shoes, "hay muchos zapatos" I would use "muchos" because we're talking about more than one shoe, and shoe in Spanish is a masculine noun.
I do not know where you got necesita and necesito from. These are not nouns but "verbs." The change in the ending is not because of gender but because the subject is different. For necesito the subject is "I," for necesita, the subject is "he/she/it." The first one means "I need" the second means "he/she/it needs." Perhaps you were thinking of what is called the "past participle." The past participle for necesitar is "necesitado." The past participle can be used like an adjective, and thus it must fallow the same rules as all adjectives, it must agree with the noun, or subject, for example: "Soy necesitado" means "I am needed." However, if it was more than one person you would say "somos necesitados." "We are needed." However, should the gender of the subject be female you would say "soy necesitada" or "somos necesitadas."
And some words are automatically feminine or masculin like biblioteca. You wouldn't change that one. How do you know when/what words to change or do you just have to memorize which words are masculin/fem?
Most nouns are always one gender and never change, the only exceptions I can think of for changing of gender for nouns is when it applies to people. Professions and the such.
Adjectives don't have a gender, well, I guess I don't know enough to really say that, what I mean to say is that they don't have a fixed gender. But fortunately they are easy to figure out, you'll just have to figure out what the adjective is at the base and then conform it to the number and gender of the thing it modifies.
For your last question, yes, you are going to have to memorize which words are masculine and which are feminine. There are patterns, that can help you out, like most words that end in "a" are feminine, though as Lazarus pointed out, many of the exceptions to that rule are in common words, and " there are less than 10 feminine words ending in -o, meaning that over 99% of the words [ending in O] are masculine." But for the most part you're left with just having to remember which words are feminine and which are masculine.
There are patterns, that can help you out, like most words that end in "a" are feminine.
That's true, over 90% of the words in -a are feminine, but you should also mention that that means a few hundreds of "exceptions" for this pattern, many of those words are common ones, like "día". On the other hand, there are less than 10 feminine words ending in -o, meaning that over 99% of the words with this ending are masculine. The latter is a much safer pattern.
A Dictionary will tell you. M for masculine and f for feminine for nouns.
Necesitar is a verb, thus necesito and necesita are conjugations that have nothing to do with gender.
