How do I set the input file directory in a arcmap geoprocessing tool?
If you always want to use data located in the same file, you must set this file as current workspace in ArcMap. In the Menu, go to Geoprocessing > Environment, Expand Workspace > select the file you want to use as a workspace. The workspace can also be a geodatabase. See Current Workspace (Environment setting) for more details about the current workspace and how to customize it.
This allows you to type only the basename of the dataset (e.g.: points.shp). The complete path to the data will be updated when you've finished typing (and click elsewhere). As @artwork mentions, if you click the browse button to select a dataset, you will land in the last browsed folder.
To set home directory, select Catalog Option button (in ArcMap)>Home Folder and browse for new home folder. Note for this update to take place you will have to close and re-open the application. Finally, open desired tool click parameter browse folder icon, and click the "Go To Home Folder" button and it will re-direct to the defined home directory. It seems that the "Go To Home Folder" button is only available in geoprocessing tools when opened within ArcMap.