Rotation

Billboard: A 2D image rotated in 3D so that its normal follows the normal of the viewing direction.
Another definition would be: A 2D image that is rendered on the same plane as the viewing camera in a 3D world.

Without knowing what math your using in your system Ill just throw out some psudo code:

//get the two normals we are considering

Vector particleNormal = normalize(particle.normal);
Vector cameraView = normalize(camera.view); //(default camera in opengl is looking along (0,0,-1) )

//do a healthy amount of error checking here, see if the two normals are already equal, or close enough that you can ignore it

//we use the dotproduct to figure out how far out of line the particle actually is

double temp = dotProduct(particleNormal,cameraView);
double angle = radiansToDegrees(acos(temp));
//we will need to rotate the particle around an axis orthagonal to both the particle normal and view vector

Vector fixAxis = crossProduct(particleNormal,cameraView);
fixAxis = normalize(fixAxis); //not sure if this is needed by opengl''s glRotate

//now just rotate it using the angle we found around the axis we found

glRotatef(angle,fixAxis.x,fixAxis.y,fixAxis.z);

As for the dotProduct and crossProduct, a trig book or a quick search on the net can fill that in. The acos() is the arc cosine function found in math.h.
The formula used for the angle is:
(assume vector1 and vector2 are unit vectors)
cos(angle)=vector1 dot vector2
thus: angle = acos(vector1 dot vector2)

In wrote this really quickly and didnt test anything but it should be generally accurate.