Well now, that depends on how far back you want to go. Let's start in the 1800s.
Somalia wasn't really of any importance to the modern world until after the opening of the Suez Canal in 1867. The British recognized the stategic importance of Somalia as a resupply depot for troops travelling between Britain and India. At this time, Somalia was mainly used to resupply meat (cattle and goats) for the British troops.
In the 1880's the French moved into Somalia after British forces ejected the French from Egypt. The objective of the French presence was to maintain a single coaling station on the Red Sea to support their naval forces traveling between France and their colonies.
In 1888, Italy realized that they should get a bite of the pie before it was all gone. The Italians put forth a lot of effort to try to develop large sugar cane and banana plantations.
When Somalia finally gained independence in 1960, the Southern region had been controlled by the Italians for the previous 72 years, and the Northern region had been controlled by the British. The British failed to properly establish a political system, whereas the Italians had spent the last 10 years ensuring that a puppet government was firmly established. Naturally, the Italian backed government quickly dominated the political arena.
The puppet government eventually failed due to corruption, fraud, lack of public support etc. In 1967, public unrest really started to escalate, and on 15 October 1969, the Prime Minister was assassinated.
Enter Major General Mohammed Siad Barre. Although the movie Black Hawk Down portrays General Barre as some savage, unorganized mob leader, he was actually a pretty smart guy who was educated in Italy. Now, Barre quickly took over and began conducting massive reforms. These reforms included both good and bad things. For example, there was a 100% increase in the rate of primary school enrollment, but there was also a massive increase in the number of executions without trial.
After establishing his new country, the Soviet Union was one of the first to recognize the new government. This of course meant that Somalia was going to get plenty of arms and ammunition from the Reds. Barre then fell in love with the Soviet's sense of nationalism. He wanted to create the same sense of nationalism in Somalia, and this is really where things start to fall apart.
Somalia consists mainly of nomadic tribes. These tribes don't recognize borders, and they don't really give a damn about who's in charge. Barre wanted a massive nationalist movement and industrialization of Somalia. Any tribal leaders who refused comply were murdered... and then all the women of their family were raped. As you can guess, this caused a certain degree of... unrest.
In 1973, a bunch of oil was deposited in the border region of Somalia and Ethiopia. Barre quickly assembled his forces to secure the region, but the Soviets decided that the Ethiopian government was more stable, so they gave the weapons to the Ethiopians instead. This resulted in a very one sided fight that left 500,000 somalis displaced.
Years of drought and famine followed, and pretty soon, the tribes banded together in an effort to oust Barre. And that just about brings you up to the opening scene of Black Hawk Down.
Sources:
Nannini, Vance J, Decisions in Operations Other Than War: The United States Intervention in Somalia, (Fort Leavenworth: U.S. Army Command and General Staff College, 1994
Metz, Helen, Somalia: a country study, (4th ed; Washington D.C.:1993
Farer, Tom J., War Clouds on the Horn of Africa: The Widening Storm, (2d ed; New York: Carnegie Endowment for International Peac, 1979