Portugal began establishing small coastal colonies along the coast of Africa in the late 1400s. The Dutch, French and English soon followed suit. The site of El Mina was a classic example of these early European colonies. Africans had iron weapons and herds of cattle. They had built up resistance to European diseases like small pox and were not that far behind in weapon technology. Africans also had better resistance to tropical diseases like malaria, which killed off Europeans more rapidly than the natives.
It wasn't until European medicine developed a way of dealing with malaria and invented more modern weapons like machine guns and pack howitzers that Europeans could impose their will on inland African states. How the Europeans came to colonize most of Africa is detailed in the book "The Scramble for Africa: White Man's Conquest of the Dark Continent from 1876 to 1912" by Thomas Pakenham. It looks at all of the European colonies established in Africa in the late 1800s.
A more narrow focus on the dense jungles of the Congo is provided by the book "King Leopold's Ghost: A story of greed, terror and heroism in colonial Africa" by Adam Hochschild is probably better able to answer your question, without distracting you with the politics of South Africa, Cecil Rhodes and the struggle between France and Great Britain to control the upper Nile river valley.