In order to obtain a work contract with a Spanish employer, you will need a NIE number first. To get the same rights as a Spanish employee you will need a work contract that is at least 6 months in duration.

You must then register with the Spanish Security office (Seguridad Social) and for this you will need to take your work contract and NIE. Any spouse, partner or child is also entitled to medical cover. An additional “beneficiary” form must be completed, and the Marriage Certificate, Certificado de Convivencia and Birth Certificate(s), as appropriate, should be produced with the other paperwork.

Working mothers with a child under 3 years of age can apply for child benefit, by completing and presenting Form 140 "Deduccion por Maternidad" to the Hacienda (tax office).

You will receive a temporary card that shows your social security number. This card should be taken to your local social security clinic and you should register with a doctor. This will instigate your permanent health card(s).

Working for a UK Employer in Spain

If you go from the UK to work in Spain for a UK employer, having been previously ordinarily resident in Great Britain, and expect to spend no more than 12 months there, you would normally continue to pay employee’s Class 1 National Insurance contributions in the UK. In all other circumstances Spanish Social Security contributions as an employee would be payable. To access the Spanish state healthcare you should obtain Form E106 from the Department for Work or the Inland Revenue in the UK.

Source:BlevinsFranks

See also

The Self-Employed System
Pension Entitlement
Working in Spain