Renting out your property in Spain comes with the same risks that you might find in the UK: You have the opportunity to make money on your investment, but you run the risk of running into problem tenants along the way.
Landlords now have the possibility of asking long term tenants to pay their the community fees and municipal taxes associated with the property. But this condition must be clearly stated in the contract and you should have proof that the tenant clearly understands the arrangement, otherwise they can take you to court - and win the right not to pay these fees – leaving you as the owner of the property liable for this.
You are also legally entitled to - and recommended to take a deposit. You are entitled to ask for one month's rent in advance for Unfurnished accomodation and two months' for furnished accommodation and offices. It would probobally be best for you to collect this deposit yourself for safekeeping until the contract ends, however, you could be asked to leave the deposit with a neutral third party (Like a Rental Agency) or a Government Body.
You can manage the rental of your property yourself, or you can put it into the hands of a property management company. There are many such companies in Spain, and this is an occupation that many expats fall into on arriving in Spain - check references thoroughly as numerous problems can arise when dealing with a less than reputable company.
Finally, by law you are supposed to declare all rental income, but many properties are let on the black market with only verbal agreements between landlord tenant. Be advised that should you choose this route, a contract still exists between you and your tenant in the eyes of the law, and should any disagreement arise, your tenant need only prove payment of the last month's rent in order to initiate a complaint against you.