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Abhijeet, the first thing you need to decide what kind of a car you want to design sport, sedan, SUV, truck etc..., Then you need to decide on the size by looking at the cars in the category, let us say you need to design a super sport car then is it front engine or rear engine if you want rear engine then look at Lamborghini, Ferrari, Bugatti, http://www.freewebs.com/superautoworld/statsonsomecars.htm , then you start by placing the driver position starting with the H point http://www.carbodydesign.com/archive/2009/03/04-h-point-automotive-design-guide/ then you place the occupants and concede the ergonomics (same attached link), then you need to decide what kind of structure and materials you like to use chassis, space frame, tub... http://www.keithwakeham.com/Files/ChassisDesign.pdf , then you move to the suspension design http://niumotorsports.com/formula/Learn/Suspension/Suspension%20and%20Frame%20Design.pdf. Please let me know if you need any farther help.