Do I need to become a sole proprietor to become a social media marketer? I already have my sales tax ID.

If any individual is engaged in any type of commercial or business activity, he/she has already been a sole proprietor.

Sole proprietorship firm is nothing but an individual carrying business. There is no specific procedure for formation of this organisation. But, it can surely be registered, as you have already obtain sales registration.

Make sure you consult a professional who can guide you if any other formalities are required for your business.