In its simplest form:

CREATE PROC dbo.createnewdb (@dbname SYSNAME)

AS

BEGIN

DECLARE @sql NVARCHAR(MAX) = N'CREATE DATABASE ' + QUOTENAME(@dbname);

EXEC(@sql);

END

GO

EXEC dbo.createnewdb N'newdb';

There are too many considerations to list here before creating a stored procedure like this and allowing folks to call it but something like I showed should get you started.