I think there are a few tutorial threads on progress reports, but I'll give you a little overview of how they work.
If you want to track kills (attacks would be easier, I think) you need to first declare the var. I'll use "Attacked" in this because that is what I would use.
Then whenever you attack a yak in your script, add "Inc(Attacked)" right afterwards:
Simba Code:
if AttackYak then
Inc(Attacked);
The Inc() procedure will add a value of 1 to the var that you use as a parameter. So essentially, every time you successfully find and click a yak, the "Attacked" var will increase by one. To add XP to this, all you have to do is multiply the times attacked by the amount of XP killing a yak gives.
Simba Code:
Writeln('We have killed: ' + IntToStr(Attacked) + ' yaks!');
Writeln('We have gained: ' + IntToStr(Attacked * 400) + ' xp!');
{Replace 400 with however much xp you gain}
Hope that helps