you need to start Sql Server using Trace flag 1400 inorder to configure mirroring. You can use the below command to perform the same,
NET START MSSQLSERVER /T1400 and then proceed configuring mirroring. you to apply SP2 for both the servers in order to avoid using trace flag.


if you need to configure you can enter the above Trace flag in services itself, use the below steps.

 

        Goto RUN --> Type sqlservermanager.msc

        Right click on SQL Server(instancename) service and click on properties

        Click on Advanced tab

        In the startup parameters enter this ;-T1400 and click on OK

        Restart SQLservices and then try configuring db mirroring

you need to start Sql Server using Trace flag 1400 inorder to configure mirroring.

Sorry to say that above is incorrect. The trace flag is needed ONLY before SQL 2005 SP1 when mirroring was not supported. Any version after SQL 2005 SP1, mirroring feature is supported by enterprise edition. No need of trace flag.

As Shanky said, please provide little more details about which specific trace flag you want to know more.