Generally the HEC-HMS inputs are prepared in GIS environment. If you are using ArcGIS you can download the HEC-GeoHMS. Its an extension in ArcGIS that you can use to prepare inputs for HEC-HMS. The You will need inputs in HEC-GeoHMS like DEM, land use map, soil map to create a model that you will later import into HEC-HMS. The extension has a step by step manual to guide for the process.