One of the biggest omissions in Japanese textbooks, classes and one-on-one lessons is gendered language. Ignore it and at some point you will wind up sounding like a little Japanese girl — or a guy — when you didn't intend too.
What exactly is "gendered language"? This refers to how males and females speak differently from one another within a language. It is a feature of other languages (Spanish, for one), but the Japanese version differs as it refers to gender roles and is not "grammatically gendered" — meaning that if you are a boy and speak like a girl, there is nothing grammatically incorrect about it. You would just sound like a girl, and that's no fun.
So how do males and females speak differently in Japanese? The good news is the difference is not actually that great, and learning Japanese gendered speech patterns (onna kotoba, otoko kotoba — women's words, men's words) is more about being aware that they exist than about memorization and study. A key difference, though, is sentence-enders. For example, girls use the sentence-ender わ (wa), whereas boys wouldn't use anything at all. Take the word 高い (takai, tall). A girl might say 高いわ (takai-wa), but a boy would just say 高い (takai). Another key difference is the way guys and girls refer to themselves and others. 僕 (boku) is a masculine way to refer to oneself, and あたし (watashi) would be a lot more feminine. There are so few differences, it's much easier to provide a gendered-language cheat sheet.