Every German noun carries one of three genders, shown by its article: der (masculine), die (feminine), das (neuter). In the plural all three become die. The gender is a property of the WORD, not of the thing it describes — das Mädchen (girl) is neuter, der Löffel (spoon) is masculine. There is no reliable way to guess it, so the article has to be memorised with the noun. A few endings are dependable: words ending in -ung, -heit, -keit, -schaft, -tion are die. Words ending in -chen or -lein are das. Most others you simply learn. ein/eine is the indefinite article (a/an). It has no plural — for plurals you use no article at all.
Persian has no grammatical gender and no definite article — «کتاب» is just book, and definiteness comes from context or را. So there is no habit to fall back on here: der/die/das is genuinely new machinery, and guessing from meaning will mislead you. Practical rule: never write a new word in your notebook alone. Write «die Wohnung», never «Wohnung».