When a new word is not in your head yet, go through four steps. None of them is a guarantee, but they usually lead to the right form faster than trial and error.
1Look at the end of the word-ung, -heit, -keit, -schaft, -ion, -tät give -en, -in gives -innen, -chen and -lein leave Ø. With der and das words in -er, -el, -en the form usually stays the same: Lehrer, Fenster.
2Check the articleA word with die almost always takes -(e)n: die Frau → die Frauen. The well-known exceptions: Mutter, Tochter, Hand, Stadt.
3Ask whether it is a loanwordMany words from English and French (Handy, Auto, Hotel, Café) take -s. Words in -um and -ma from Latin and Greek you learn separately.
4Only der and das are leftMost often it is -e: der Tisch, das Jahr. If the word is short and neuter, it is often -er with an umlaut: das Buch, das Haus. After that, only the dictionary helps.
How the plural is written in a dictionary
Next to a noun, dictionaries and textbooks give a short note. The dash stands for the word itself: Tisch, -e means Tisch + e. A dash on its own (Lehrer, -) means the form does not change, and ¨ means umlaut.
der Tisch, -eder Stuhl, ¨-edas Kind, -erdas Buch, ¨-erdie Frau, -endas Auto, -sder Lehrer, -der Apfel, ¨-
Make flashcards with the plural from the start: der Tisch on one side, die Tische on the other. That way the form sticks together with the word instead of becoming a separate fact.