翻译：今天我第一次上学

对啊
还可以说：It is the first time I go to school today.

This is my first day in school.
Today is my first time in school.

你的英语说法不符合语法
中文也很奇怪，加个“是”比较正常：
今天是我第一次/第一天上学

你这样的翻译显然是不对的，典型的chin-glish（中式英语）。我觉得你可以这样翻译：
It is the first time that I go to school today.
Today is my first time to go to school .

It is the first time that I go to school today.

Today,I go to school first of time.
晕 你小学？

It is the first time that I go to school today.