Tax registration with the authorities :
Bulgaria
Tax registration with the authorities
If planning on staying/working/buying a house or car, you must obtain a Social Security Number (NSSI). This number identifies you to the Bulgarian taxman and is required when dealing with the tax office. Details regarding how to obtain one of these can be found in the link below.
Bulgarian National Social Security Institute
If you are employed, your employer should sort your tax registration out for you, however if you are self employed, you will need to find an accountant who can help set up your business correctly in a way which will legalise you with the relevant tax authorities.