You may start your CPL course when you have 150 hours flight time, you have completed your class training for the aircraft you will test in and you have reached your 18th birthday.
One other thing to consider is that the night qualification must be completed, if not before the start of the CPL course, then during your CPL training. This must be completed before the CPL LST.
A valid Class one must be held throughout the CPL training.
You can start the CPL course before the exams have been completed or started! This I believe is not wise. You MAY NOT attempt the CPL or the IR skill tests, unless ALL of the relevant exams are passed. If you have not completed the exams I suggest this should be your priority.
To start your IR course is similar. Your class rating training must be complete before the start of the course, although one major school to my knowledge has been exempt this and my school was absolutely denied it!!!!!
You need 50 hours cross-country, 10 hours of, which must be fixed wing aircraft. Be over 18 years old. Hold a valid class one or two medical.