In order to enter Canada as a visitor, you will need to apply for a Temporary Resident Visa either in person or by mail at a Canadian Embassy, Consulate or High Commission. Each country has its own specific requirements. Amongst other important requirements, you will need to demonstrate your ties to your home country and proof of funds. Both single-entry and multiple-entry visas are available.
As long as you can demonstrate, with strong documentation, that your intention to remain in Canada is for a specific purpose and for a temporary period of time, your chances of obtaining a visitor visa are high. You can visit Canada for many different reasons, such as tourism, business, visiting friends and family members.
If you have been refused a visitor visa in the past, you can apply for another one, but it is recommended that you submit proper documentation as your previous refusal will be in your file and this might affect the outcome of your new application.
Once you are in Canada, if you wish to remain longer, you have the option of extending your stay, again, if you are able to meet the requirements of a visitor.