Whenever you are calling another country outside of the UK the international prefix is always 00. This is then followed by the code assigned to that country; in this case the dialling code for Canada is 1.
If you are calling Canada from a mobile phone in the UK, then the 00 is replaced with a + symbol. So it will be +1 then the city dialling code followed by the phone number. If you have a Smartphone the + symbol should be obvious on your keypad, possibly above the O. If not and it is not immediately obvious where it is then you may need to try pressing and holding 0 or the * to get the + symbol.
If you go to the Country Codes website mentioned above you can get a $5 free prepaid phone card and really good savings on international calls. To call Canada from the UK using a prepaid phone card you will need to dial the access number on the card and then when prompted enter your PIN number. Then all you need to do is follow the instructions above for calling an Canada from the UK using the dialling code 00 1.