In his "I Have a Dream" speech, what does Dr. King advise the oppressed black Americans who became victims of unfair treatment to do?

Dr. King offers many pieces of advice to his followers in the quest for civil rights. One of the first is that his followers must not jeopardize their "rightful place" with "wrongful deeds."  Even if their opponents meet them with "physical force," they must respond with "soul force."  King believes that enough blood has been shed and lives have been taken and that the way forward is through nonviolent protest and peaceful insistence on equal rights.
Dr. King also advises his followers that it would be a mistake to "distrust all white people."  He understands that much of the power in America at this time rests in the hands of whites, and that many whites support integration and equality for all Americans; thus, they are allies.
In addition to reiterating his commitment to nonviolence and building alliances, Dr. King encourages his followers to remain committed to the cause until their aims are realized. He tells his audience literally and figuratively to keep the faith.

The main advice that Martin Luther King, Jr. gives to African Americans in this speech has to do with the idea of nonviolence for which King is famous.  He urges his people to make sure that they keep the moral high ground in their struggle for civil rights.
King is famous for having believed in the idea of nonviolent protest.  He thought that was the only kind of action that could actually gain African Americans the rights that they wanted and deserved.  He believed that nonviolent protest would convince white people that the people who wanted to keep blacks from having equal rights were on the wrong side of the argument.  Therefore, he urged African Americans to continue to use nonviolence. 
King tells African Americans that they “must not be guilty of wrongful deeds” in their attempt to gain their rights.  He said that they had to make sure that they did not let themselves give in to their anger over how whites had treated them. As he said:
We must forever conduct our struggle on the high plane of dignity and discipline. We must not allow our creative protest to degenerate into physical violence. Again and again, we must rise to the majestic heights of meeting physical force with soul force.
Thus, we can see that King’s main advice to African Americans is that they need to continue to fight for civil rights, but that they need to adhere to the tenets of nonviolence so.