By plane
Fly into Marseille or Nice. Marseille is nearer just south, but budget airlines such as EasyJet prefer Nice. There is a navette shuttle bus that can take you from the airport to the bus station near the center of town.
By train
Aix has both a TGV and a regular station and is well connected both to the Paris - Marseille line and via Marseille to the Genoa - Nice - Barcelona line. The same shuttle that runs from the airport to Aix also services the Aix TGV station. The regular train station is at Place Victor Hugo...a five+ minute walk from the center of town GPS coordinates: 43.522562, 5.445303.
By car
If you come from anywhere up north you will most likely use the A7 motorway Route du Soleil that runs from Lyon to Marseille whether you come from Switzerland or from the UK via Paris. At exit 27, take A8 La Provencale to Aix. From Spain, you'll take A9 La Languedocienne, then turn onto A54 through the city of Arles you might consider stopping there, then turn onto A7 and A8 respectively, as described above. From Italy, just take A8 passing Nice and all the posh Côte d'Azur resorts.
Avoiding toll roads can be slow but highly pleasurable, many of the routes nationales offer wonderful scenery that motorways can't deliver save maybe some parts of the A8 east of Nice.
Parking in Aix can be quite difficult. There is at least one free car park but allow plenty of time to find a space and the spaces can be quite tight to get into!.