Bus 35 takes you in 15 minutes to La Joliette for € 1.5 per person (one way). The bus stops at the exit of port N°4 where the big cruise ships dock. There are 3-4 buses per hour. From La Joliette you have 2 options. You can walk to the city center at Vieux Port or take the connecting bus 55.
Walking is preferred considering you also pass along the Cathedral which is located about halfway. The total distance from La Joliette to the center is about 1 km.
How do I get to the bus stop from the cruise port of Marseille?
From the cruise terminal at Port N°4 is a green road marking. If you follow this you will arrive at exit 5 of Port N°4. It's about 1.5 km walk. At the exit of the port follow the road uphill. At the top of the viaduct is the N°35 bus stop.