Once you arrive there are blue P signs directing you to the lots around town, the 1st couple we tried were already filled but we eventually found an outdoor one with a spot open. Payment was at a meter than only took coins, so we didn't have to worry about it, we paid until 19:00 when the meter was done for the day. Down the street there was a garage that took credit cards. We were visiting out of season so I'm guessing that these lots all fill up in July and August, our guidebook said Parking des Gorguettes was free with a shuttle bus that costs 1€.
There is a bus service from Cassis to Marseille which takes circa 45 minutes and costs 2.70 euros. It leaves from Le Gendarmerie, not the casino which is the terminus for the SNCF station and some park and ride facilities. Get the timetables from the Tourist Office near the beach and le port.
The bus terminates in Avenue de Prado in Marseille near Castellane metro station which is quite a walk from Le Vieux Port and St Charles SNCF station. The view from the bus travelling into Marseille is spectacular.