How can my line items always appear with the last record first.

I want to see the last input in the head of a column instead of being sorted by the FM File number

If you're talking about line items in a portal, go to Define Relationships and set a sort order for the relationship.

No, I mean in my main database, in List Mode (for exemple, my Client DB). Everytime I open this DB, I would like the last client to appear at front...

You'll need a field that you can sort by, an auto-entered serial number would be best. Sort descending, then create a script with the Sort(restore) step. Now set that script to run when you open the database, or else assign the script to a button, or include the script in other scripts that go to the list layout.

An alternate method would be a script that always takes you to the last record when you open the database/go to the list.

YA method would be to use a portal as I mentioned originally. This is actually a common practice, so worth a try if it suits your purposes.

OK, I already have that button for scripting, but I thought it was of a kind of preferences set up.

But it's kind a if i put it i an opening script.