You first need to look at the relationships between your tables. Unless the correct relationship with the correct match fields is in place, there's no easy way to get this to work for you.
If you open the Invoices starter solution that comes with either FileMaker 11 or 12, you can see a working example of what you want to do.
You may also find this thread to be helpful: Auto Fill
With regards to your second question, we achieved that result for our system so that when a status field on a "receipts" table changes to "Printed", only users with a full access privilege set can edit the fields in the record.
See "Editing record access privileges" in FileMaker Help and check out this particular sub section: "Entering a formula for limiting access on a record-by-record basis" for a description of how to set this up.
Hey, Never got around to saying thanks!
Im still working on checking the relationships of my tables but, looking at the starter solution was a good idea!