Genders are usually determined by the sound of the ending of the word. There are always exceptions, but for example, words finishing in "ET" are masculine (like un objet, un navet, etc.) and words in "ETTE" are feminine (like une crevette, une assiette, etc.)
It's better to learn words with the article "un" (masculin) and "une" (féminin), as "le" and "la" have a flaw : if the word starts with a vowel, we use l' regardless of the gender - therefore you won't learn the gender for all words starting with a vowels (and there are plenty of those).