If Nigeria is your home country, you cannot apply from there. You have two options: (1) Obtain a tourist visa (TRV), travel to Canada and claim asylum once you are here. To be approved for a TRV, you will have to demonstrate strong ties to your home country to show you have no plans on remaining in Canada long terms. (2) Leave Nigeria and move to a different country. Once you are in that different country, be officially recognized as a refugee by UNHCR. UNHCR will then decide where you will be resettled. It most likely won't be Canada.