Milan has many different companies letting you rent a vehicle and pay as you go. You can normally park those vehicles on the blue and yellow lines but always double check limited traffic zones (read here for more info on how to drive around the city).
If you already have one account in your county with the same company, like Share Now, it will work in Milan too.
Car sharing services:
Scooter sharing:
Bike sharing:
If you want more info on how to roam the city using public trasportation (best option) have a read here.