I am dropping off my rental car in Amein - wanting to take the train or bus to Paris - then the Tgv to Montpellier
my question - what train tickets do I buy?
If I'm right in assuming you mean Amiens, then you would take a train from there to Paris Gare du Nord. Take the RER D directly from Gare du Nord to Gare de Lyon. It's a ten-minute ride, there are 3-4 trains/hour, and the fare is €1.60.) Take a TGV from Gare de Lyon to Montpellier Saint-Roch.
For a zoomable map of the Paris Metro and RER system, go here.