I am also from Africa doing a PhD in the UK. You will have to think about funding, there various organisations that you can approach. You've then got to send in your applications to as many supervisors, having located a good research topic e.g from this website, I found mine here. Hopefully you will be invited for interviews. I think the biggest issue will be your funding, if you have that sorted its great. My research comes with a studentship for an african student.. so the best advise its to just search and trust in God to provide. God bless you.