Welcome to PPRUNE.
It's unclear as to which exams you are referring to, but given your wish to do a PPL, I presume you mean the PPL written exams?
If you are talking about the ATPL, or CPL exams, then you must already have a PPL before you can start.
The PPL exams are no big deal - a few weeks of hard work will see them completed. I think it's best to do these before you do the PPL, but you can do them alongside the training as well.