This is a little information about Employment here in Spain, I will work on the basis that you are an EU resident, as there are different conditions for non EU persons.

EU residents do not need a work permit (permiso de trabajo) to work in Spain. EU nationals can enter Spain as a tourist and register with the Spanish national employment office (Instituto Nacional de Empleo - INEM) to look for a job. You then have 90 days to find employment - you can obtain an extension after that date or leave Spain and re-enter for a further 90 days.

Types of work permit

Employees
Type A: for seasonal or temporary work, valid for nine months and not renewable.
 
Type B: it enables the foreigner to work in a specific profession, activity and geographic area, valid for one year and renewable.

Type C: issued after the B permit has already been renewed and has expired. A C permit is unrestricted, allowing the foreign employee to work in any job anywhere in Spain. Valid for three years and renewable or can become a Permanent work permit.

Self-Employed (autónomo)
Type D: for self-employed people in a specific field of work and location.
Valid for one year and renewable for two years.

Type E: issued after the renewed D permit has expired, this entitles the foreign worker to operate in any profession, including self-employment, anywhere in Spain. Valid for three years and renewable - or worker may be issued with Permanent work permit.

Self-employed non-EU nationals must show they are investing about €120,000 in Spain to start a business or that their professional activity will produce a profit and benefit to Spain, for example by employing Spanish (or European Union) workers.

Either employed or self-employed
Type F: for workers who cross over the Spanish border every day from their usual country of residence. Valid for five years and renewable.

Essential work permit documents

Employees

•Passport.
•Medical certificate.
•Certificate of criminal records issued by the authorities of your home country, except when it was presented upon application for the visa.
•Three passport-size photographs.
•Fiscal registration number (NIE or CIF) and the Social Security registration number of the employer.
•Offer of employment containing labour conditions.
•Full description of the job and the company's activity.
•Proof of the employer's solvency could also be required.

Self-employed non-EU nationals must show they are investing about €120,000 in Spain to start a business or that their professional activity will produce a profit and benefit to Spain, for example by employing Spanish (or European Union) workers.

Either employed or self-employed
Type F: for workers who cross over the Spanish border every day from their usual country of residence. Valid for five years and renewable.

Self-employed

•Copy of your valid passport
•Certificate of criminal records issued by the authorities of your home country, except when it was presented upon application for the visa
•Official medical certificate
•Three passport-size photographs
•Full description of the job and the company's activity
•Proof of professional qualifications or licenses if applicable, or registration to the Spanish Social Security system, or your NIE.

Here is a basic guide to the law about working in Spain

EU nationals

If you are an EU national you can enter Spain as a tourist and register with the Spanish National Employment Office (Instituto Nacional de Empleo - INEM) to look for a job. You then have 90 days to find employment. You can obtain an extension after that date or leave Spain and re-enter for a further 90 days. Once you get a job, an employment contract will be necessary to apply for your **residence permit.

Employment contracts

The employment contract must contain the following provisions:

• Details of the employer and the employee
• Over what time period the contract is in effect, including date of starting employment.
• Type of contract
• Professional category
• Description of work conditions: work location, hours, schedule
• Specification of any trial period
• Duration of the holidays
• Compensation
• Collective bargaining agreement applicable

Both parties must sign the work contract. The employee should be provided with one copy of the contract previously stamped by the corresponding National Employment Office (Oficina Nacional de Empleo -INEM). 

Working conditions

Jobs in Spain must be performed under, at least, minimum working conditions, which are described in the Statute of Workers (Estatuto de Trabajadores) and any applicable collective bargaining agreements.

The standard work week is 40 hours, although this varies depending on the occupation. The standard weekly uninterrupted rest is one and a half days (two days for minors), though this also varies from one occupation to another.

Spanish labour rules prescribe that daily working hours are limited to nine hours of work per day. It is also prescribed a minimum of 12 hours rest between working days. 

Overtime (horas extraordinarias) is also restricted by law to 80 hours per year, unless collective bargaining prescribes something different, for which the workers must be compensated either by economic compensation or by paid time off in lieu. It is illegal for minors to work overtime.

There are 14 national holidays (two of them are locally established) and the vacation period legally paid is 30 calendar days, unless otherwise stablished in the collective bargaining agreement.

Working hours are closely connected with other aspects of employment such as salary, the amount and structure of which is normally established by the applicable collective bargaining agreement. Normally, there exist two extra payments — in June and December. 

Salary deductions

The employer must deduct monthly social security contributions from the employee's salary, provided that this exceeds the minimum annual salary, which is around EUR 421 a month. 

These contributions are calculated as a percentage of your taxable income — currently 6.4 percent. The employer must enter your contributions in the Social Security General Treasury (Tesorería General de la Seguridad Social). 

Your salary is also subject to personal income tax in Spain, and your employer will deduct monthly amounts from your salary to enter it in the National Tax Administration. The amount deducted will depend on your personal income.

These are only general guidelines and not definitive statements of the law. All questions about the law's applications to individual cases shall be directed to a Spanish lawyer.

Information supplyed by David SR. Vinalopo Trader Forums.