mORMot Open Source

Well, with the good old xitami web server I did something like this:
program summing;
{$APPTYPE CONSOLE}
uses
  SysUtils, cgi;

var
  x, y, z: double;
  tempStr: string;

function sum(a, b: double): double;
begin
  result := a+b;
end;

function FormFloat(s: string): double;
begin
// read the name and the value of the input data from a HTML form sent by the browser
// and return the value as a double
end;

begin
  x := FormFloat('name1');
  y := FormFloat('name2');
  z := x+y;
  tempStr := FloatToStr(z);
  // send the data to the browser
  writeln(tempStr);
end.
What I want is to do is, more or less, something like this using mORMot.
Seen mORMot is huge, what are the important parts to learn first to write this kind of applications? Thank you.