Would this town be on the regional train line as I cannot find it on sncf.com?
It is only 1 hour trip outside of Paris.
If so you would just go to the station and buy the ticket?
As has been noted, if you can't find it on SNCF then chances are it's not a direct enough run. Check bahn.de, which will give you most connections, even if they are indirect. Look for the last transfer city/town to your destination, then plot your sncf search into two legs.....you'll at least be able to purchase a ticket for the first part, then simply grab a ticket for the regional leg when you arrive at the end of the first section.
At most times of the day you will travel on Banlieue (suburban) trains. Even though they're operated by SNCF, they don't appear on voyages-sncf.com. Service is frequent and cheap and you'll need to make one change. The bahn.de site has the timetables.
Trains leave regularly from both gare du nord and st. lazare...to Pontoise...where you would change for direction Creil, getting off at Auvers. On sat and sun until(thru) Oct 28, 2008, there's a direct train from gare du nord that leaves at 9:56 am and returns at 18:15pm...35 mins one way. Might seem like a long day, but you'd have time to visit everything leisurely and not think about train connections
Yes, you can buy the ticket at the station. For a complete timetable, visit Transilien, the suburban arm of the SNCF. Scroll down to Réseau Paris Nord H. Select "Creil-Pontoise" and click "Télécharger." Your computer will download a PDF of the timetable. LV=weekdays, S=Saturdays, D=Sundays and holidays. This trip costs 1.50E each way. Pontoise-Downtown Paris = 4.95E each way.
You can print out timetables for the other half of your trip, too. Using the same site described above, scroll down to RER C. Gare du Nord-Ermont Eaubonne-Pontoise should already be selected. Click "Télécharger" to download.