There are no airports that service Aix-en-Provence. The nearest is in Marseille which is a short bus or train ride away.
Check the SNCF website for schedules and prices on trains and make sure to validate your ticket before boarding the train.
You can go to Check My Bus for information on buses.
If you’re taking a day trip, sometimes it is the most convenient to mix and match the bus and train based on schedules.
A common day trip but you could stay a day or two if you get out to the surrounding areas as well.