Where do i start getting more HP out of my Impreza My99?

 

A full decat exhaust is the cheapest option, then possably an Ecutek ecu upgrade, both these will add to your insurance though.
I would start first of all by recommending that you sort the brakes out first, atleast change the disc/pad combination, if not changing the discs/calipers to something that stops the car
Be warned, once you start, it will get expensive as you wont want to stop

Tony