The best official route is to set up an S.L company equivilant to a UK Ltd, set up cost will be 3,006 euros plus any solicitors or notary costs, you will need to submit your chosen company name plus 2 alternatives to the mercantile register, takes about 4-6 weeks, alternatively as in the Uk you can buy a non trading shelf company, much quicker and a little cheaper however you will have to chose a name from the companies already registered. you can however just chose any company available and “trade as”
Spanish employment laws are a little!! complicated find yourself a good Asesoria and discuss whether its best for you to be an employee of the company or to be self employed. Obviously if you are an employee of the company you will have a contract drawing a salary, making it easier for credit etc.
Whatever route you choose to take the minimum social security cost will be circa 240 euros a month, you will immediatly have medical cover, unemployment benefit after 12 months, and qualify for a Spanish pension after 15 years contributions.
Dont forget insurances, liability etc.
Hopefully the link will help to get your qualifications recognised.
Not condoning it but I guess this is why so many people work on the black, at least until they get established.
Hope this helps