If your have an older car and it seems to have been taken care of, but it does not run right, there are several things you can do to check over the vehicle and see what can be done, without spending a lot of money. First, it could be something as simple as your transmission fluid is low, or your transmission fluid and filter needs to be changed. Second, have the compression checked on each of the cylinders. The technician should be able to advise you if it is within acceptable levels, generally you want the psi to be close to each other, but not more than 15 psi. Third. If it is old, it is likely that the spark plugs have not been replaced, or the distributor cap and rotor. If the car is new enough and does not use a distributor, then the fuel injectors may be dirty and adding an injector cleaner to the fuel may help clean them out. The shaking could also mean your timing is off a little bit. It could also mean your spark plugs are not firing correctly and you need to replace the spark plug wires too. These are repairs and checks that you can do yourself, which would be fairly simple to do. If your car has white smoke out the back while driving, it can be much more serious. It can mean your engine needs a new head gasket, or intake manifold gasket. Hopefully this will head you in the right direction. Good luck.