2000 Toyota Corolla Horsepower

If you are talking about mods, then you need more air in the engine which will allow more fuel which will make more HP.

(1) Higher flow cylinder head (larger valves), larger throttle body, modified intake manifold, larger displacement (bore and stroke).

(2) Or, install turbocharger, supercharger or Nitrous Oxide system.

1 will require a reprogrammed PROM and possibly larger injectors. A turbo or supercharger will also require a new PROM.

Nitrous (a wet system) uses the existing fuel pump to add gas to the engine.

Whatever you do you should have the car run on a chassis dyno with a wideband O2 sensor to make sure your air/fuel ratio is correct.