If you are new with Rietveld refinement, maybe it's better to try with some tutorial. In these examples the refinement procedure is well explained. In general in the first step you should vary the scale factors, background coefficients and correction parameters, then the lattice constants, atomic coordinates and thermal and occupancy parameters. These last ones with great caution because are correlated and you can easily found wrong values. This is a standard procedure that can be adapted to your specific case.