Why does my car vibrate alot at start up & at idle?

I have a 2008 Honda Civic & it vibrates ALOT when I start it in the morning (avg 75°F) & when at idle at a stop light. It vibrates less when running, but it comes back when I stop at a red light. It vibrates a bit more when it's on PARK. I would like to know a list (as much as you know) of all the this that can cause it to vibrate, thanks.

Just a quick question, would changing spark plugs really help reduce alot of vibrations, or just a little?

Also, so you know how much it vibrates, when I leave a bottle of water in the cup holder, it creates medium/large water ripples.

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