You would automatically have been given Canadian citizenship for being born there. Your mother would not have had to renounce your Canadian citizenship to get you a British one since the UK allows dual. You would not have lost the Canadian one through taking the UK one because Canada allows dual too.
Nobody gives you dual. It's more like your one grandma gives you a book and your other grandma gives you a book and then you have 2 but not because somebody gave you two, because you got 1 from each place. Countries that don't allow dual would be like the grumpy grandma who says "since you already have a book, I will not give you one unless you return the one you have" but UK and Canada are both nice grandmas who don't care how many other citizenships you already have.
So either apply for a passport directly or if you prefer to get a citizenship certificate first, you can do that at
http://www.cic.gc.ca/english/citizenship/proof-how.asp but you normally don't need a citizenship certificate when you are born in Canada and have the birth certificate to prove it.