These kinds of questions would be better answered by an accountant who can first get a little insight into your business, and provide some tailored advice. (Perhaps the questions themselves aren't, but if they're necessary to ask, then I think you need an accountant's help). 

I think this kind of help is a little too comprehensive for a general discussion forum - I'm not saying there isn't anyone here who could walk you through all the basics of business operation, a lot of people could, but in my humble opinion, the vast majority wouldn't, because that's getting into highly time-consuming billable-service territory, to be perfectly honest.  

As a very rough guide, you pay tax on your income after all expenses. You don't just sell 10 items for $100 and then work out 10% of the total to calculate tax, unless you incurred $0 expenses in the operation of the business.

Income tax isn't 10% though, and GST is something different entirely, not to mention a more exhaustive (if not comprehensive) process, so if you have to be including and paying that, consulting an accountant becomes imperative.