train from Aix-en-Provence to Arles

You can check the schedules on the Deutsche Bahn website.

All such questions can be answered on the DB website: https://www.bahn.com/en/view/index.shtml
This is technically the German Railways website, but it lists all national railway companies throughout Europe, but only sells tickets to or from Germany.

For a random weekday in April it gave me:

Aix-en-Provence depart 09:47, 1 change Arles arrive 12:03
Aix-en-Provence depart 10:10, 2 changes Arles arrive 12:10
Aix-en-Provence depart 11:48, 1 change Arles arrive 14:01

Lots or earlier and later trains, I just gave you an excerpt. You can look up your desired date and time.

You can also just check the SNCF (French national train service) website. https://www.oui.sncf

You can put it in English. This will also give you any bus options that may be run by them (there would also be regional busses, but those would be found via region-specific websites). Train schedules are published about 3 months out, with ticket sales being available at the same time. If your time frame isn't posted yet, you can look earlier to get an idea of what trains typically run--the schedule should be the same for the same day of the week for all the standard trains (i.e., the trains aren't posted for Tues. June 19 yet, but I can look on Tues May 29 and expect things to be similar--one caveat is if it happens to be a holiday). If you happen to be going in the summer, there will likely be more trains/routes added.