how to keep car slow when doing parking's 

Slipping the clutch
So you’ve got the hang of the bite point, next it’s time to learn how to slip the clutch. Slipping the clutch is an essential skill if you want to pull off manoeuvres such as reverse parking or three point turns. Luckily, slipping the clutch isn’t that difficult.
To practise, all you’ve got to do is find the bite point and let your vehicle move forward very, very slowly. Try to control the speed of the vehicle with slight adjustments to the clutch; depress it by a centimetre or so to slow down and release it a similar amount to speed up. When making very small adjustments like this, it’s helpful to imagine something very delicate, such as an egg, underneath the pedals. Apply very gentle pressure and try not to crack your imaginary egg.
While you’re trying not to crack the egg that’s under your clutch, maintain gentle pressure on the accelerator. Don’t be tempted to try and use it to speed up and slow down, use the clutch instead.
Once you’ve repeated this little exercise a few times, clutch control will become second nature. You’ll be slipping the clutch like a pro every time you do a manoeuvre!
Once you’ve mastered clutch control, you’ll be able to control your car at low speeds, making manoeuvres such as reverse parking much easier. All it takes to get the hang of it is a flat and quiet road and a bit of patience. You can tackle clutch control in one of your driving lessons or you could find a safe place to practise with an appropriate co-driver.