kizwiki's guess is certainly possible but it is just a guess.
What you have to wrap your head around is that putting the car in gear puts load on the drivetrain. It is that load that is leading to the shaking.
This might happen because of a bad motor mount.
It will also happen for 100 other reasons that no one can guess about because you haven't said anything else about the car. Most of these reasons have to do with simple things like spark plugs - so now that you've gotten that far do the rest. New wires, new air and fuel filters. A check for vacuum leaks and a check of the fuel pressure would be in order.
Basically, rather than guessing first at a motor mount I'd guess first at your car not running right.