There are a few effective ways to start any speech. First of all, a speech should always begin with a hook. A hook is a catchy beginning that grabs the listener's attention and sets the focus for the speech. There are a few interesting hooks.
One is anecdote. Tell a short little story of a hypothetical or real person suffering from the genocide.
Another is to give a shocking fact or statistic. I am sure that all of the statistics about genocide are shocking. Find one that speaks to you, and begin your speech with that. For example, since the war started _____ million people have died in the violence in Sudan, _____% of those were children. If we apply those statistics to the people in this room, the first ____ rows would be gone.
Ask a question. This is a common way to start a speech.
Begin with a quotation. This can be from the news or a relevant quote from a famous person related to the issue at hand.
A few other general strategies to capture attention in a speech include asking a rhetorical question and using an apt quote. The rhetorical question seems too weak for this particular topic, but you may be able to think of something with enough impact to make it work. A quote would work, perhaps a poignant statement from a survivor regarding the horrors of the events he has lived through or a statement of condemnation for these actions or some other quote directly and dramatically leading your audience into your speech. Interesting topic!
Have you considered opening with an anecdote? You could briefly tell the story of one of the Lost Boys of Sudan. There is a lot of information about them, and their story is fascinating and heart wrenching at the same time. Many people enjoy narratives; so anecdotes are an attention-getting way to open a speech or essay,
An effective speech grabs the attention of the audience right away, just like a good newspaper article or TV show. When trying to be informative on something as shocking, horrible and ugly as genocide in Darfur, attention getters can be a little tricky.
You might start by trying to establish some perspective as to the number of murders we are talking about here. When you say the number 500,000 (I realize the number may well be higher, or you probably have a more accurate one), it's a little impersonal, hard for the audience to grasp. If you say "How would Americans react to an armed group murdering every man, woman and child in Portland, Oregon?", it gives them a mental image to understand and relate to, and a concept of the massive scope of this crime for you to build the rest of your speech on.