Two of the major messages in Hansberry's A Raisin in the Sun are to never let go of dreams and to recognize the importance of family. Lena Younger wants to help make her late husband's dreams for his family come true, so when his life insurance money arrives, she makes sure that part of the money goes into securing a home for the family. Rather than use the entire sum for the house, she divides the money equally among the family, giving a portion to Beneatha for school, and the other portion to Walter to do as he sees fit. Walter, of course, acts unwisely, and he allows himself to be blinded by greed. But Lena encourages the family to pull together in the face of this tragedy and tells Beneatha that people need love most when they are in times of hardship. In the end, the family maintains its bond, and they move on together to the next chapter in their lives.
There's nothing wrong, of course, with reading something in order to find a life lesson, moral, or message, It may work for simple stories (even then, though, sometimes things aren't as simple as they appear), but that approach doesn't really do justice to complex literary works.
A Raisin in the Sun has a number of themes. In addition to what the previous poster has stated, I would add that there is very clear and sustained treatment of the conflict between generations of a black American family. This conflict comes to light in the discussions of money and religion, in particular.
As I see it, one difference between a moral/life lesson/message and a theme is that the first can be boiled down to a single statement that almost always ends up sounding like a cliche, whereas the second often focuses ideas but doesn't reduce them to a simple statement that we've already heard again and again.