The first step in assigning the absolute stereochemistry is to assign priorities to the groups attached to the chiral center. Higher priority is given to higher atomic number. Then rotate the structure so that the lowest priority group is away from you (into the plane of the screen). Then, determine whether the priorities for the other three groups descend in the clockwise or counterclockwise direction. If the group priorities are in a clockwise order, the chiral center is of R stereochemistry. If the priorities are counterclockwise, the center is S.
For the compound shown here, the highest priority group is the oxygen (red). The second highest priority group is nitrogen (blue). The third highest priority is carbon (grey), and the lowest priority group is hydrogen (white). With the hydrogen in back, the priorities are in a clockwise direction, so this compound has R stereochemistry.