When I place Reaver for computer,it doesn't do anything...
How can I force it to build scarabs,and attack?
Use the Set unit hanger trigger to give the unit the scarabs. It is for interceptors for carriers too. There is a tutorial on this somewhere...
Kaboom is right, except its' Modify Unit Hanger Count.'

J00 gotta make sure that the comp has the cash to build the reavers or interceptors etc. Otherwise, modify the unit settings as well.
Also, to make it attack, there should be a trigger for it. It's Order units.
Are you sure about needing the minerals/gas for the Scarabs? I'm pretty sure the 'Modify' Action doesn't require anything, like the 'Create' Action (You don't need minerals/gas to use the Action 'Create Units.'
It has to have a enemy unit to attack.
You don't.
The build time can be very high and cost a lot but it will just keep appearing for the reaver. It is like spawning units with triggers, that does not take resources to make.