There are numerous companies ready and willing to ship your belongings to Spain. You should obtain quotes from at least three as prices vary greatly.
Shipment within Europe should only take a few weeks. From anywhere else times can vary but your belongings should be with you within two months. If you can be flexible about arrival times you may be able to greatly reduce the cost of shipment by sharing a delivery.
Customs clearance is no longer necessary if you are shipping your personal belongings between EU countries. If your belongings are arriving from a non EU country you must enquire about customs formalities in advance. Your shipping company should take care of the paperwork and ensure that all documentation is correctly completed for you.
Do insure your belongings while in transit and also while in storage prior to departure or upon arrival. It is advisable to use an independent insurer rather than one provided by your shipment company as it may be easier to make a claim. Typically, insurance costs one to two per cent of the declared value of your goods.
Before shipping your possessions check that furniture will fit into your new home in Spain. Only bring with you that which you cannot do without in order to minimise shipping costs and remember that Southern Spain has some gorgeous furniture shops!