The first place to start is your DB. Think about what sort of information you need to put into it. Presumably, you'll want a few tables, one for cars and their details (colour, make, registration plate, etc), one for customers (name, address, etc) and one to link what car went with what customer. Keeping things in different tables like this makes it extremely flexible (imagine a customer brings a car back within the allowed period and it gets sold on to another person, with 3 tables you can keep a record of that easily)
After that, you need to write the code that manages the data. Break it down into sections on paper, and group those that are similar if you want. For example, add and update are quite similar tasks, so they might share some aspects of the code.
This is all probably stuff you'll need to show in your assignment anyway.
