if you're just jacking up one corner/side, then under your car there should be these circular looking spots. one just inward of where the front wheelwell ends, and one in the rear just inward of the rear fender.
if u wanna jack up the whole front or rear end w/ a hydraulic jack (not the sketchy wind up jack in the trunk) then this is what cha wanna do:
front, if you have enough groud clearence jack it up under the front subframe (just behind the oil pan). Place the jack stands on the "frame rails", they are rails that are part of the unibody hehind the front wheels about a foot in.
rear, jack it up under the rear subframe, just in front of the differential. place the jack stands on both sides of the subframe towards the outside, just beside the subframe bushings/mounts.
(i actually copy & pasted most of that from two posts by Stu McHenry from e3s
gotta give them credit... i pretty much knew that, but i wanted to get it 100% right & he knows what he's talkin bout when it comes to e30 tech stuff)