T2125 is for business operation without setting up corporation, it is part of your personal tax return T1;
If you set up a corporation, you have to register. If you just want to continue a business, you might not need to, you can just use your own name to carry on a business.
Then the income goes to your corporation, your corporation need to file a business tax return, T2, (T1 is your personal tax return) and it is your corporation to claim business expense to deduct.
There are various factors to consider whether to set up a corporation, and tax is one of them.
I wish this can be helpful.