You will need a COR if you want to register for Taxumo. So that means, you have to be a registered taxpayer. Note: having a TIN is different from having a COR (Certificate of Registration or BIR Form 2303).
For a Freelancer, the process is much easier as you usually just have to transact with the BIR. Note that the process does change between RDO's so your mileage may vary.
Here's a guide written by Ginger that details how to register as a freelancer:
Now if you're registering your business as a Sole Proprietor, it's a little bit more complicated. Fortunately, Ginger also wrote a guide on how to go through the process:
Hope that helps!