Hi Ismana,
It depends where you are from. If you are a European citizen, working in Spain is not a problem. In that case, you just get yourself a NIE number from the nearest national police station, which costs around 7 Euros. With that, you register yourself at the Social Security office and get a SS Number. These two documents you need when applying for work and get a contract. A normal contract initially lasts for three months, and includes a 15 day trial period. These contracts may be half time or full time. (20 hours pw or 40 hours pw) and more often than not states the basic wage only, (1040 Eur PM full time) and if you earn more, the rest is usually paid in cash when you receive your wages. In the catering industry, your wages also include a meal per shift. Tips are normally paid in cash and depends on how the employer orfganises that.
People outside the EU will find more obstacles, especially now that the unemployed figures go up. A Spanish consulate will give all the information you need.
Working behind the scenes is sometimes still possible, but very risky for both parties. Employers cannot take on any emplyees without a contract and without coverage in the SS system. If there is an inspection, fines up to 60.000 Euros are the norm per employee without papers. For the employee the risk is not being covered when he/she is ill, not getting paid, getting fired at will, and in some cases deportation if an inspection finds out.
Working on contracts can be covering a period of one year, after which an employee becomes an Indifinido employee. Temporary contracts do not have to be renewed by both parties.