Forcing data have to be netCDF (standard CF formatted) or GRIB format. Minimal resolution in time recommended is 6 hours. Several datasets (NCEP, ERA and CCLM-data) are available at DKRZ for members of CLM-communtiy (Data at DKRZ).
An example forcing data set has already been copied to your computer system if you followed the installation procedure.
Before running a simulation you need to define your simulation domain and get the external data (parameters for orography, land use, soil, etc.) for that model grid. The WebPEP on the cclm-community webpage can provide you with those external data for your domain.
An example external data set has already been copied to your computer system if you followed the installation procedure.