The objectif of my project is to bilud a seringe pump using a unipolar stepper, Arduino mega , keypad and LCD display.
the problem on me is that I have to input the volume and time using the kaypad and the program have to calculate the speed to make the stepper motor turn to realise the perfusion.
I'm begginer and I don't know what to do 
I need your help ... can any body help me to make the programe?
It seems to me there are at least 3 separate parts to your program - reading the keypad, displaying information on the LCD and making the stepper move.
I suggest you start by doing each of these on its own in a short program. When you know how to do each one separately it will be time to start joining them together.
Have a look at Planning and Implementing a Program
I assume you mean a small 28BYJ stepper - which I have no experience of. But there are probably hundreds of Threads in the Forum about them.
...R
thank's Robin2 ... I will try my best