Find an online broker that can accept you, do the forms (application forms, verify your identity and so on) and invest.
Nigeria isn’t a “black listed” country like some places, where hardly any brokers will accept - Somalia for example. There are plenty of options.

However, it depends how much money you have to invest.
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Let’s keep this simple.

If the amount of money you have is pretty small and/or you can’t get the money out of Nigeria easily, then I would stick to local providers.

In comparison, if you have at least $500 a month to invest, or a decent lump sum, I would consider international providers.

Most people with money in the country want to diversify internationally and there is a reason for this.

It is safer and better.