sp_fulltext_catalog
is what you're looking for.
From Books Online, if you were looking to rebuild (this a drop and create) a full-text catalog named Cat_Desc in the AdventureWorks database...
USE AdventureWorks;
GO
EXEC sp_fulltext_catalog 'Cat_Desc', 'rebuild';
GO
If you wanted to start full population of said catalog...
USE AdventureWorks;
GO
EXEC sp_fulltext_catalog 'Cat_Desc', 'start_full';
GO
If you wanted to perform incremental population of said catalog...
USE AdventureWorks;
GO
EXEC sp_fulltext_catalog 'Cat_Desc', 'start_incremental';
GO