All fast trains to Avignon will leave from Gare de Lyon in Paris so you need to plan to get there - as you would expect many buses and metro line 1 and 14 stop there.
What you need to know about Avignon is that Avignon TGV is outside of town and Avignon Centre is the train station in the town itself. When you take the TGV from Paris to Avignon you will need to switch the local train for 10 minutes or so to get into Avignon Centre to get to the city.
If you book a ticket to Avignon Centre in the SCNF website you will get both tickets Gare de Lyon > Avignon TGV > Avignon Centre and this is the easiest thing to do. The trains to the city leave from the same level as the fast trains so you just cross the platform, get in the next one to the city and ride a couple of minutes. The station is right across from the city gates into the town and makes a nice entrance to the city itself.
Many people getting off at the TGV station will be transferring to the city train so you can probably just follow along - it's easy.
SCNF is the official train website and App for the French train company.
Have a great trip,
=Tod