Apart from scholarships that will be offered to you from the universities itself after you apply to them, there are external scholarships that are offered to international students who wish to study in the US.

But, first, let's talk about scholarships that can be offered from the universities itself. For example, UCLA has the following information on scholarships on their financial aid page:
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Therefore, it becomes important to decide which universities you will be applying to. Use this Grad School Finder by Yocket to see the list of safe, ambitious and realistic universities to apply to, after which you will be able to check the official scholarships offered by the university itself.
http:///2wFAJlp

Now, for the external scholarships, here's a list of scholarships you can find that are open to international students, especially from Nigeria, along with information on the entire process of studying in the US!
http:///2MtXqDN

Hope it helps!