Remember that more is not always better in the case of sound systems. Adding too many speaker in your car will make the sound louder, but if the audio is not coherent, it will produce a very distasteful effect on ears. Generally the audio system of the cars consists of one head unit with one or two amplifiers, one pair of front speakers and a small subwoofer unit. Head unit should have at least one audio output port, and it should be connected with the amplifier. Do research on the front speakers as they are main sound producing components of your system.
Rear fills are not recommended as sound should have only one output source in a vehicle. However, in few cases, installing rear fill does improve the quality of the sound. It needs to be handled very carefully or it will ruin the performance of the complete audio system.
Each speaker should play only those sounds for what it is designed. For example, a tweeter should play high pitched sounds, a mid speaker should play mid frequency notes while the bass speaker should get the low frequency notes. Filters or crossovers are used to assign specific frequencies to their respective speakers.
Some head units have a built in filtering mechanism, while for others a cross over needs to be installed between the head unit and the speakers.
The location of the front speakers actually determines the entire sound image. The distance between the right and left speakers with the listener should be same or it will not be pleasing to ears. It is not possible to perfectly balance out the distances, but effort should be made to keep the difference as little as possible.
Get good quality cables and install the complete assembly in the car. Make sure that no connection of the cable is weak, particularly cables coming to and from the amplifier. Put everything in its place and start the sound system.
A little fine tuning may still be required to some components of the system. For example, the power gain of the amplifier and tweeter sound may need to be adjusted to your taste.