The best way to start is to just jump in! To start off just choose your 'Pick-up Location' dates and times. As you become more familiar with it, you’ll find there’s plenty of in-depth functionality to help you find the perfect car hire. For example, you can now choose to drop off your vehicle at a different location by checking the ‘Return car to a different location’ box.
One the results page, you’ll have the option to filter your results. You can filter by ‘Car Class’, ‘Fuel Policy’ ‘Car Type’ and much more.
Once you select your vehicle you will be transferred to the site of the travel provider. There then may be more options available to you, so you can customize your vehicle even more.
You might find this article useful - http://goo.gl/c9EDlU