A good reversing camera can be an invaluable aid for parking or reversing your car but a bad one can be positively dangerous if the display is unclear or, even worse, if there is a time lag in it. There is no point looking at an obstruction that has appeared, if you have already hit that obstruction several seconds ago! So, a good quality camera and display, with a fast refresh, properly fitted into your car, is essential.
There are two main components; a camera and a display monitor (or you may be able to connect into your head unit if you have one). You then need some way of getting them to communicate with each other. This can be done by cable or wifi, and each has advantages and disadvantages.
Running a cable is the most positive way of doing it but there is more work involved to install the cables and keep them out of harm's way. A wifi comnnection is much easier to install but there is more chance of signal interruption or time lag. If you don't have a head unit I would personally buy a kit rather than buy the parts separately, since you should not only avoid compatibility issues but you could read reviews on the kit beforehand - an absolutely essential first step in my opinion.
A full set of instructions should be with the kit, hopefully in clear English and not Chinglish. Usually the numberplate is removed then the camera is mounted so that it will be just above it, with the mounting bracket below it. However you mount it, though, make sure it is the right way up; an upside down monitor display may be very difficult to rectify! The wires from the camera are then connected to the wiring to a reversing light.
How you connect to these wires is a matter of choice. There is a DIY system called a scotchlock, sometimes called splice connecter, which clips onto a cable, and when it is squeezed a small moon-shaped piece of steel cuts through the insulation and makes contact with the wires. Many electricians refuse to use these however; there is a chance of cutting some wire strands, steel is not a terribly good conductor, it may corrode rather too quickly and you cannot easily check that you have made a good connection. Soldering the wires together before fitting a heat-shrunk sleeve is the favourite of many but most people don't have a soldering iron. Personally I prefer to use a junction box secured to the car body with double sided adhesive tape.
There is usually a mounting bracket supplied which enables you to mount the display monitor on the dashboard or windscreen. This should be placed where you can see it easily without having your view of the road obstructed. Many are fitted with sticky pads but I prefer the vacuum type which last longer, are easier to move, and which don't leave messy marks behind.
Usually it will be plugged into your cigarette lighter socket or USB socket. If you have an 'always on' socket make sure you disconnect the plug if your car is to be parked up for a while, to avoid the danger of a flat battery.
A common problem is a voltage drop. You will need an inexpensive relay to solve this: there is a useful explanation on this website.
Get a good auto electrician to fit it for you. Many Halfords branches can do it for around £80.