These days there exist more then 1000 exchanges around the world. Some are big some are small. Some reliable, some will make you loose your deposits. More and more exchanges require you to verify your identity, from adding a telephone number to sending them a copy of your passport. Keep this in mind and check the rules of the sites before you make a deposit, and consider if you really want to send a copy of your passport to an exchange in Nigeria.
A few exchanges that are good these days are listed below. But it is not a guarantee that nothing can go wrong. And remember an exchange is to trade coins, not to store your coins because you are to lazy to install a wallet.