my car shakes when idling but when accelerating it stops

Typical problems causing excessive vibrations when idling:

• worn out engine suspension. The engine is mounted on rubber pads, which wear out and need to be replaced eventually.
• damaged dual-mass flywheel (applicable to some modern engines)

Typical problems causing uneven running, which can be also be described as "shaking/vibration"

• damaged or dirty throttle/throttle sensor/idle speed nozzle/anything that's involved in regulating idle rpm
• damaged component of ignition system: cables, coils, distributor, etc (sparkplugs too, but you've already replaced them)

Bottom line: IMHO you've already done what was possible to do without professional help - replaced plugs. You can visually inspect rubber pads for damage, that is if they're accessible. Fixing any of the above usually requires professional workshop. If the problem is uneven running, you can try checking out the throttle, but in modern engine there is more risk you accidentally knock something out of alignment than to actually fix the problem.