Gromacs

To whom who know gromacs from a beginner. 

I built *.itp file in the Automated Topology Builder (ATB) http://compbio.biosci.uq.edu.au/atb/index.py?tab=home_tab for a new molecule, so now I have two files: *.pdb and *.itp. 

Now I want test this single molecule by running the Brownian dynamics simulation. 

A which step (simplest) has to be  the next? 

mdrun or grompp? 

Give please command string for this case.

#gromacs