PHP - Help creating a PHP number generator [on hold]

I am working on a school assignment that currently needs me to build a random number generator.

The user types in a number, and the program has to create that many random numbers between 1 and 20. It shows all the numbers, together with the average, and also keeps track of how many times each number has been used in total.

I do not need anybody to write the complete code for me, I'll be able to find the syntax and the ways methods work, but I have no clue where to start. What do I use? a For loop? an if or a return, an array? nothing seems to really work.