There are two types of rentals in Spain: Short term and long term. Short term rentals are basically Holiday Lets and the contract is headed with the phrase "Por Temporada", which literally means "seasonal". Long term rental contracts state "Vivienda", which means "Residential" and are for those who are planning to make their home in Spain.
As a tenant, it is important to understand the difference between these two types of rental properties and their contracts as your rights will be different. For example, if you are planning to stay in Spain for a few years at least and you sign a short term contract for nine months, you could be asked to leave when the contract ends.
However, if you know that you intend to remain in Spain for the foreseeable future, and you sign a "Vivienda" contract, you will legally have the right to renew for five consecutive years. During this time, rent can only be raised to accommodate inflation.
You might be asked to make a deposit when you rent, and this is to be expected in accordance with Spanish law, which permits the landlord to ask for one month's rent for unfurnished accomodation and two months' for Offices and furnished Accomodation. To protect yourself you can ask that the deposit be held by a neutral third party such as a Rental agency or a government office called the "Consejería de la Vivienda".
If you hold a long term rental contract and feel that you are not being treated fairly, visit the nearest municipal consumers' office (OMIC - Oficina Municipal de Información del Consumidor). Here you should be able to present your case to someone knowledgeable in consumer affairs and obtain advice. Personnel at these offices can also help you to take complaints through the appropriate channels. To help them help you, bring your rental contract with you and any receipts or other documents that could be of use.