The basic relationship looks OK if you always rent one car to a given customer at a time. If you wanted to add the ability to rent multiple cars in one rental transaction, you might need another table so that your current join table becomes the line items table of a rental invoice.
And there are usually multiple items that can be charged as part of a single rental such as gas consumed, mileage, insurance premium, etc. So you might want that added record even if you only rent one car at a time so that you can better itemize the details that make up the rental transaction.
Also i have created a Layout based on Customer table. i wanted to make drop down list of all Customer names to select from and a Portal to add cars. but both of them don't work.
Once I created a few records in the Cars and Customers tables, both drop downs and the portal worked as you designed them to work. But I think you'll want to add that rental invoice table.
Given the value lists that you are working with, you might find these exploration files of interest:
They are free to download.
Adventures in FileMaking #1 - Conditional Value Lists (includes details on how to set up a basic field based value list)
Adventures in FileMaking #2 - Enhanced Value Selection (what to do when a simple value list won't cut it.)