How to get from Marseille to Avignon

You can check it yourself by going to bahn.de for schedules or the French rail site (SNCF) for schedule and fare information. Train from the airport to Avignon about every 30 minutes and takes about 45 mins via TGV.

You can take a Regional train from the Vitrolles station near Marseille airport to Avignon Centre, there's a shuttle bus to take you from the airport to the station. Takes about an hour, about 17 euros each.
https://www.capitainetrain.com/search/

The regional trains are cheaper, don't require reservations, and run direct from either Marseille St. Charles or Vitrolles Aeroport Marseille station. You can buy tickets in the stations. I wouldn't bother with a more expensive, reserved TGV from Marseille St Charles station, since the travel time is not faster and it also requires a connection (usually at the Avignon TGV station, which is less central than the Avignon Centre station).

See also Buying European Train Tickets and How to Look Up Train Schedules and Routes Online.

My go-to site is always rome2rio, it's much easier to navigate and comprehend. When it's time to buy tickets, it takes you to the appropriate national railway site.

Thank you so much for the responses. A great help!

An alternative route is a shuttle bus from Marseille Provence airport to the Aix-en-Provence TGV station (not centre of the town) to be whisked by fast train to Avignon TGV station. That's on the outskirts of Avignon and there is a five-minute rail connection to the old Centre station where the regional trains arrive. The regional train may be slow and rattletrap but it will do the job with a little less complication; the shuttle from the airport to the regional station is five minutes versus around a half-hour to Aix TGV. The Marseille airport website is helpful on ground connections: http://www.marseille-airport.com/index.php/access-car-parks/access/by-train-or-by-bus

Be aware that at Vitrolles Station there may or may not be someone in the ticket office. We arrived from the U.S. at ten a.m. on a Saturday into Marseille, took the shuttle to the station and then had to buy tickets from the machine there. You will need a chip and pin card (chip and sig may work, don't know) if the ticket office is not staffed, which was the case for us. After flying all night it was a little challenging to get our tickets worked out, but we did. We also took cash from and used our card to buy tickets for a young American couple who were not prepared with a card that would work in the machine-they would have been in a fix otherwise.
We first took a ride to Arles and spent a few hours there before heading to Avignon Centre. It was a good way to see a bit of Arles and make something from a travel day, be outdoors while fighting off jet lag, and letting some time pass so when we got to our B & B in Avignon it would be time to check in-about 6 p.m. If this is of interest, there is a good spot to leave luggage in Arles near the station: Hotel R’egence, 5 rue Marius Jouveau, 13200 Arles, supervised baggage storage 07:30-22:00 for a couple euro per bag.