A country becomes an imperial power by conquering other nations and through extermination of undesired people. Another quality of an imperial power is unequivocal exploitation aimed at the benefit of a small handful.
Doc, I wonder if you just wanted to know what "imperial" means? It's the adjective for empire. So to become an imperial power you would need to conquer other nations and bring them under your control. The usual method is warfare but it can be done to a certain extent by economic means, by making another country dependent upon you or on the goods and services you're able to offer them.