"When creating a table with a unique ID Number, how do I increment automatically starting from a number other than 1?"

You can append 100 to the ID field, and the next number will be 101 and will continue from there. After you've added the 101 record, you can go back and delete the 100 record.

If you've got Table1 and the autonumber field is ID: Create a new query, don't add any tables, but switch into SQL View and paste in this, and then run it:

INSERT INTO Table1 ( ID ) SELECT 100 AS Expr1;