What training is available in Nigeria? I have no idea.
Read the posts here on ifixit AND the answers, probably a third of them are people having problems after they've botched a repair, learn and study how they broke their phone so you don't do the same. Another third of those questions are people asking things that anyone that has done repair very long has seen and can answer quickly, these are also good to read as they'll teach you things you don't have to discover yourself in trial and error.
read every internet source you can find, research problems you come across until not only you understand what happened and how to fix it, but WHY it happened and how to not do that again.
youtube has a lot good resources as well as a bunch of garbage, its hard to decipher which is which until you've got some experience to know when someone is full of **** or when someone is showing a way of doing things that are reckless. A good rule of thumb is if someone is disassembling a phone with a suction cup, it might not be a good resource. If someone is using those little short stubby junk tools in the video that come free with ebay parts, it might not be a good resource.