There are probably etymological reasons having to do with where the word originally came from, but that's hardly helpful. There's no obvious logic behind it, you just have to learn for each word whether it's m or f.
Generally, words ending in o tend to be m and words ending in a tend to be f, but there are exceptions.
Since this uses a short form possessive adjective ("mis"; primera persona plural), normally it would be omitted when written or spoken if the possessor is obvious. For example, you wouldn't say "Me encanta mis vestidos" (I love my dresses); instead you will say "Me encanta los vestidos" (I love my dresses) only if the context is clear that you possess that dress. Remember, everything in Spanish is all about context.
Saludos y buena suerte (:
Hablo inglés y hablo español por lo que me preguntas especial sobre inglés. Por cierto, ¿Quién más esta de acuerdo en que necesitan más idiomas?
I speak English and Spanish so ask me anything, especially about English. By the way, who else agrees that they need to add more languages?
I know you posted this a year ago, but I figured I should answer anyway:
When the word ends with a vowel, you add -s but when it ends in a consonant you add -es. Also, if the word ends in a z ( I don't think there are any at this point) you drop the z and add -ces. Ex.: Lapiz (pen) plural is Lapices