Well, since "Hello World" isn't really convenient or simple for robots, let's try a "move forward" program
This program turns on the motors and set the robot moving forward
// this program assumes that your
// motors are attached to "A" and "C"
// OUT_A is the macro for port "A"
// on an RCX, OUT_C is port "C"
task main {
while(true) {
OnFwd(OUT_A + OUT_C);
}
}