I have read that to say that something is something, for example It is a student, you use the hiraganaだ. 学生だ.
Can I say 私の弟学生だinstead of 私の弟学生です？I dont know when is used "da" は 日本語 で何と言いますか？

質問を翻訳

We have two writing styles. Those are a direct style(だ、である) and a desumasu style(です、ます).
The direct style is literal. Because it sounds arrogant in conversation, it is difficult to use for daily speech. This style is good for describing.
On the other hand, the desumasu style is one of polite way of saying. We usually use 敬語(keigo/ an honorific) especially for others( strangers, elders and betters) except our family, friends and close people in daily conversation. This style is safer to writing and speaking.
At the beginning of Japanese study, if you think it's hard to learn those two styles, I recommend you the desumasu style for the above reason.