I am making a roboy model at the moment. I seem to have a problem and can't find any info about it.

As rviz loads the model it is viewed in default position. For me this (should) mean(s) i see the back of the robot. 

Now i need to know how i can change the "RPY" values. I'm trying to rotate a shape like you would turn your head from left to right. For 90 degrees that is. Wich value (r,p or y) should i change and to what? I can't seem to get it right.