Keep all your reciepts, keep all your bills. If you are a sole proprietorship, there is no formal paperwork so long as you declare your income as business income on your tax returns. Learn about all the tax deductions you can make for things you use in your business (interest on home, all your rent, utilities, maintenance, use of car, fuel, etc). Keep an auto log and all fuel, maintenance receipts if you do use your car. Most things used completely or partly for business can be tax-deductible (clothes and shoes are one exception - so tools, electronics, books etc are allowed.)
If you plan to make more than $30,000 you are supposed to charge HST/GST (I think this got raised recently). If you are not sure, get a HST/Business number from CRA. If you do happen to make over $30,000 you are on the hook for the HST amount, even if you did not collect HST.
You can't claim business losses from your other income, but you can apply a loss in your business in one year to a profit in a future year of the same busines, therefore reducing your taxable income. So file a tax claim even if you made no money. But if you make no money year after year, CRA will probably be suspicious. Your business should exist for the ultimate goal of making a profit.
You should get a book or read all the CRA literature regarding small businesses in Canada. Welcome to the world of self-employment.