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Converting Screen to 3D World Coords

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I'm trying to write a method to convert from screen coordinates to 3D world coordinates but I don't know where to even begin. I'm working in Java with JOGL and my 3D math knowledge is very minimal. I don't know if it makes a difference but, in my game I'm using a 3rd person camera with the ability to rotate 360 degrees about the y-axis and 90 degrees about the x-axis.

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You can use the gluUnProject function. Every point on the screen corresponds to a line in the 3D world, going from the camera in the direction of that pixel. When using gluUnProject you specify the screen coordinates as well as a Z-coordinate which is the distance to the 3D point you want.
If you have drawn your scene already, and want to find the 3D-coordinate of what is visible at a certain pixel on the screen, you can use glReadPixels with GL_DEPTH_COMPONENT to obtain the distance to pass to gluUnProject.
You also need the matrices to pass to gluUnProject, which must match the camera-matrices you have set up to transform your 3D world, as well as the viewport. These can be obtained using glGet (glGetDoublev and glGetIntegerv).
Example:

GLdouble modelMatrix[16];
GLdouble projMatrix[16];
GLint viewport[4];

glGetDoublev(GL_MODELVIEW_MATRIX, modelMatrix);
glGetDoublev(GL_PROJECTION_MATRIX, projMatrix);
glGetIntegerv(GL_VIEWPORT, viewport);

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