about writing diary

"Today is the first day I go to the new school." is correct.
However, when you write down what happened for the whole day, it means that the things have happened.
So it's better to use past tense to write down.

e.g.
Today is the first day I go to the new school.
I woke up early in the morning.
I arrived school early.
I met my friends...... We were happy to share our happiness......

However, we can use present tense as the following sentences:
e.g.
Our class teacher is Miss (Chan) ....
She teaches us English...
Siu Ming sits nearby me.....
He is a talkative boy.....
We like eating snacks during the recess.....

When this is a habit or a fact, we can use Present tense.

Hope you can practice more.

英文日記中,大多數用的係past tense
不過有時會用其他tense

如你形容後來發生的事情是過去左的話
就要用past tense
如果未發生 / 未過去就可以用further tense....

eg. I knew some friends in my class.
我在我的班內認識了一些朋友 (過去了)--->past tense
I will have a PE lesson next Monday.
我將會在下星期一上體育堂 (未發生/未過去)--->further tense

希望幫到你!!!!

If me, I'll always use is/ am.

But i think you'd better use present tense............