.db is typically used by Sybase SQL Anywhere, and Sybase IQ. Since you are tasked with converting it to MS SQLServer, it's a safe bet that the file is from SQL Anywhere.
You can find out about installing, and upgrading .db files here
Once you have the software installed, and the database running, you can use Sybase Central, the management GUI, or dbunload on the command line to extract the DDL you need to build your MS SQLServer Environment.