You don't need anything to sign up to Taxumo!
Having your Certificate of Registration (aka COR / Form 2303) will help though as this will help you setup your account.
If you're wondering about how to go about business registration --
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:
To know more about Taxumo's BizReg service, feel free to visit this page:
Hope these help!