How to reduce allocated Space to DB in mssql 2005

Hi ,

We are using SAP with Mssql2005 on Windows 2008r2.server.

One week back we provided 400 GB space to My database files (for each 100GB), But right now we don't want that much space for My system.We are planing to use that extra space for another system.

I want to know below points.

1)How to reduce my database free space from Mssql 2005 management studio.

2)What is the Procedure.

3)Is it possible to reduce the Database free size once added to SQL Database files.

Please suggest the procedure, its urgent requirement.