All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
The scholars differed into two opinions in regard to the obligation of making up the prayers which a pubescent person missed without any sound reason. The majority of them are of the view that he is obliged to make them up and he is not acquitted from sin unless he makes them up. In our site in Islamweb, we consider this to be the preponderant opinion. Therefore, you should endeavour to make up those missed prayers.
As regards how to make them up while they are so many, then each day you should perform as much missed prayers as you can if this is not a hardship upon you, until you believe that you have made up all the missed prayers. You should at least make up two days of missed prayers each day provided this does not hinder you from earning your daily bread to spend on your family. Additionally, you should seek the forgiveness of Allaah as much as you can. If you repent and are keen to make up your missed prayers, then the mercy of Allaah is vast, and repentance may wipe out your negligence and abandonment of the prayers in the past if you die before making up all you missed.
For more benefit, please refer to Fataawa 95419 and 84403.
As regards the praying person saying ‘Rabbana wa lakal hamd’ (i.e. Our Lord, for You is all Praise) when rising up from the bowing position, then this is called Tahmeed, and this is one of the obligations of the prayer, whether on the person who is leading the prayer, the person praying behind the Imaam or the person praying individually. Thus you should say it before starting to supplicate.
Allaah Knows best.