First and foremost, Canada does not recognize frozen licenses. You either have the license or you don't. I will assume that in your case you have a CPL and are working your way to having enough flight experience to validate the ATPL. If that's the case, you likely don't have the experience to hold a Canadian ATPL and you're better off looking to convert your CPL. I only bring it up because lots of people come here (or come back here) wanting to convert their frozen licenses and get upset when they realize they're only eligible for the CPL (or in some cases the PPL). Just be prepared that you might not find the answer you want.
1. Read and follow the guidance in this
linked page.
2. Compare your hours against those required for the ATPL as shown
here. If you don't have the required experience, you cannot apply for an ATPL. In that case scroll up to 421.30 on the same page and compare your hours against the CPL. If that doesn't work, you may have to continue scrolling up to compare against the PPL. It happened to me coming home from New Zealand when I wanted to convert my CPL and it's happened to other people I know. The required hours are not flexible, so be prepared to do some clean-up flying so that your hours meet or ideally exceed the minimums. Also note that any supervision time you may have cannot be counted against the command time requirements. You will eventually read about a program called PICUS, but those hours are only obtained and credited when flown at a Canadian air operator with an approved PICUS program. They cannot be ported from another country or even from another Canadian operator. This is what holds up many European pilots when they claim to have the 200 hours PIC, but end up finding out that they only have 100 PIC when they remove their supervision time.
3. In either case of whether you have the experience to hold an ATPL or CPL in Canada, go do a Canadian Class 1 medical. Do not spend any money on any training or books or materials before you have the medical in hand. If you don't already hold a Canadian license, this is how you'll get your license number (file number) to sit the exams.
4. Read this
thread. It already has all the information you need to have regarding the exams you'll need to write.
Good luck.