Make sure your coolant is full. If the coolant drops below the thermo sensor, your car will run like absolute crap. There are two of them on the side of the head. If you disconnect the coolant sensor that the ECU uses, it will stall within a second. If you disconnect the other coolant sensor, the cooling fan will cut off (provided that it's on when you unplug it).

Other than that, if your idle is high, but it has stabilized... you need to check the vacuum system for leaks. The best way to do that on a NA car is by ear. Listen for hissing at idle with all of the electrical stuff inside the car (AC, blower fans, radio, etc...) off. If you don't hear anything hissing, LOOK all over the intake manifold for loose or missing vacuum lines or hard/dry-rotted vac hoses. Replace those. There's a vacuum diagram on the underside of your hood.

If you can't find anything abnormal around those areas, spray carb cleaner on the intake manifold flange where it bolts to the head, your injector seals, throttle body gasket. If your idle speed changes up or down as a result of this, you've got a vacuum leak that's allowing more into the engine than it thinks it's supposed to have, and that's what's causing your wonky idle.

If your intake manifold is leaking, the part to fix it is a cheap $5 part, but the labor certainly isn't. Sometimes people sell cars that have this problem because many mechanics charge about 8 hours of labor to fix it. It's not the easiest place to work on, but there are much worse cars to do this job on than a civic, that's for sure. I could do a civic in 2 hours, but I've done it before.