Assuming GBOOKSQL is your Database Name?
You create a table the very same way as in access. Thus it might be more convenient to use a GUI tool like Enterprise Manager. You can download a trial of MSSQL Server and just install Enterprise Manager, if your host allows that. Most hosts offer a web-based way to manage your SQL Server. Check with them.
AS for dispalying and updating records, well I suggest you read some tutorials as there is not much difference as to using Access, short of using Stored Procedures though.
SQL Server can only be as secure as the underlying OS. So it all depends how the whole env. is setup.
How fast? This depends too. How many hits, does the Guestbook have? How many records etc.