You need to continue making regular car payments, maintain vehicle registration, and then transfer the car's ownership. You cannot drive the car unless you have proper registration, and penalties will be charged if this is not done on time.
Follow the instructions found here, which vary depending on your specific situation and relation to the deceased.
You will likely also need to fill out an ITD 3337, an Application for Certificate of Title, which can be found here.
Additionally, if you need assistance dealing with the driver’s license, license plates, or disabled person placards of the deceased, click here.