You must start paying the Qadha of your prayers. Take it easy. You don't need to finish them off in the shortest time.
I'd suggest you keep a diary with tables in it and as you gradually star paying off you should check them out. This will keep you motivated and also on track with your Qadha.
Also, while paying off Qadha Namaz you can focus on the wajibats of the prayer and cut down the Mustahabat.
For further details on the wajibat of salah pls visit the link below under the Islamic laws: