Don't take CPL
My advice, through bitter experience, is to say don't do the CPL under any circumstances whatsoever: do the ATPL even if you are an old private pilot wanting to become a club instructor.
If you are a young pilot, there is even less justification to do the CPL, as it confines you to the right-hand seat if you decide to enter airline service (that's assuming the airlines will be interested in you at all, which I doubt). There is no upgrade route to convert from CPL to ATPL unless you do the whole 14 ground school exams for the ATPL.
The CPL groundschool exams have only nine papers, (as opposed to the ATPL's 14) but you are actually examined in 13 (yes, thirteen) subjects within those nine papers. The syllabus for ATPL exams contains very little indeed that is not also in the CPL examinations.
Let me explain the implications of this. Let's take an example, Navigation. In the ATPL, General Navigation and Radio Navigation are two entirely different papers each with, I suppose, 70 odd questions in the exam for each subject: a total of about 140 questions. Clearly, it is possible to pass one and fail the other. If you fail one, you only have to revise the one you failed for your next attempt.
CPL Navigation is one paper that embraces both General and Radio Navigation, but you still only get, say 70 questions within which to demonstrate your knowledge. The syllabus for CPL Navigation covers at least 85% of the ATPL syllabus for both General and Radio Navigation.
Since the pass mark is 75% in all papers, you don't have to get many questions wrong in the CPL Navigation to fail effectively two subjects. This means you will have to revise both subjects for your next attempt.
The Central Question Bank comprises a body of questions 85% of which are perfectly fair and reasonable. The remaining 15% are split equally between complete 'no brainers' and questions that require a depth of understanding far beyond anything that any commercial pilot needs for operational purposes. Questions are selected for inclusion in the paper by some kind of random selection process, so by sheer luck you can get either a preponderance of 'no brainers' or a preponderance of exessively difficult questions within those 15%.
I think you can see that even if you have genuinely done the work, just a few of those excessively difficult questions could scupper your chances of passing, and then you have to revise the whole of two subjects all over again in order to repeat one paper. I found that this made the CPL far trickier to pass than the ATPL!
Finally, I suggest you read Clive Hughes' 'Guide to Becoming a Professional Pilot'. It may look amateurishly produced and expensive for what it is, but the information in it is invaluable. It will give you the whole 'low-down' on the whole business.
Good Luck!