If you have the cif file you can use EXPGUI GSAS to run your refinement.

Here i have attached a link, which will explain the basic things u need to understand in doing Rietveld refinement and also have a practical tutorial attached with it . Go through the tutorial. This is based on a software package, GSAS.

http://www.aps.anl.gov/Xray_Science_Division/Powder_Diffraction_Crystallography/

This is a classical one million dollar question! You must start to use the tutorial that  many softwares have. You can try Fullprof, Gsas, Maud and others. But also if the refinement seems a simple tool, pay attention to the results interpretation

I recommend Full Prof Suite, i think the interface is friendly. I have made refinements of structures known in contrast to the same with any modification, so:

1. Translate your experimental dxr file to a format to be read in full prof.

2. Find the cell parameters in programs like Find it.

3. Use the tutorial to build the file to refine.

* I know how to refine, so these are the minimun things you have to be clear.