See a tax accountant if this is going to be your first time. The accountant can tell you want you can and cannot declare. Accounts tend to make conservative calculations, and can tell you if you are paying too much or too little.
If you underdeclare, you run the risk of being audited. If you overpay, you can reclaim some of the money back, or apply it to your next quarter.
You also have to decide if you want to do the regular method and keep track of your expenses, or do the 'quick method'.
Its good to hire an accountant for the first time. They may charge a bit, but it will save you the pain and hassle. Besides, you can claim GST on accounting services.
The basic rule of thumb of self-employed people is to don't underpay, keep all your receipts, don't overexpense and whatever generated income you receive, set aside 1/3 of it for taxes.