Don't know what is wrong.

I have a 2001 honda civic lx with 133000 miles on it.
when I start my car it starts fine then idles fast and then goes really low and shakes like its going to die before leveling off. The car drives fine but when I stop and idle I notice the passanger seat shakes. When it doesn't shake I can feel vibration in my seat and in the sterring wheel. I recently replaced the spark plugs and now the problem is worse. The longer idle the more I can feel something shaking in the engine. Plus there is a constant clicking noise coming from somewhere around the belts, and I swear after I drive for awhile I can smell something overheating in the engine. What could be wrong, and how much would it cost to fix it?