I want to perform MD simulation of protein-ligand complex using VMD-NAMD. When I want to create protein structure file (PSF), there is dialogue box asking me to parameterize my ligand.
Parametrization is a pretty challenging task. I'd rather suggest you to search carefully - perhaps your ligand has been parametrized yet.
Otherwise, you have some other options. There are some servers which prepare some basic parameters set using atom similarity, but they usually need some tuning, especially for charges.
There's a nice tool in VMD which supports this process (force field toolkit), but here you still need some patience and perform calculations in Gaussian.