Today is my last day for at the company.

Did i write this correctly ?

Today is my last day for at the company.
I used to work in Business planning department but these days the company is going through hard time so I had to move to another department a week ago.
But I don’t want to work as a noobie in new department so I quit my job today.

Today is my last day for the company. at my job.

Did I write this correctly ? (you did a good job!)

Today is my last day for the company.
I used to work in the business planning department but these days the company is going through hard times, so I had to move to another department a week ago.
But I don’t didn't want to work as a noobie newbie in the new department so I quit my job today.

I is capitalized in English.

Every thing is looking pretty good!

Today is my last day for at the company.

Did I write this correctly?

Today is my last day for at the company.
I used to work in the Business Planning Department, but these days the company is going through a hard time so I had to move to another department a week ago.
But However, I don’t want to work as a noobie newcomer in new the department so I decided to quit my job today.

Good job! Try to avoid beginning sentences with conjunctions. Good luck with your new job! 

Today is my last day for at the company.

Did I write this correctly ?

Today is my last day for at the company.
I used to work in Business planning department but these days the company is going through a hard time so I had to move to another department a week ago.
But I don’t want to work as a newbie in the new department so I'm going to quit my job today (or I quitted my job today, if you meant past tense!), (or from my job, both are correct).

Hope you succeed in your new job :)

Good Luck :)