It is always useful to get a certificate in English, not only to get a better job, but also because some Universities require their students to have one.
Internationally, the most appreciated certificates are the Cambridge exams. There are two chances to take them every year: May/June and December. You need to put your name down some months before the exams take place.
Here's the link that you need to register. http://www.britishcouncil.org/es/spain-exams-palma.htm
ESO: PET para jóvenes
Batxillerat: FCE para jóvenes
CAE and Proficiency are usually taken after at least two years at University and some time spent in an English-speaking country.
A cheaper option is to get your EOI certificate, highly appreciated in Spain:
You can register in February/March and take any of the exams in May/June with the possibility of having a second chance in September. (Nivell bàsic (A2), nivell intermedi (B1), nivell avançat (B2) i nivell C1)