The Selectivity Exam has 3 sections. Use of English (Grammar), Writing a Composition (Essay) and Comprehension Questions. Here you can practice each area separately by clicking the links above. And access full exams with answers in the exams section.
There are also some general selectivity pages such as Selectividad.Info which covers all communities and subjects. You will need to register your email to get use many of them but they often have a lot of useful information and many Exams and Notes to download.