The Spanish road network has some 340.000 kilometres in length, being 7.000 of those highways, pay-highways and fast ways. Most fast ways ('autovía' or 'vía rápida') are free but —specially in Cataluña and in the Mediterranean coast— there are many pay-highways.
In the main roads there are no problems to finding petrol stations. Inside these you should also find small shops and bar areas. However in secondary roads or small roads you will be better off if you carry a map with you and if you pay attention to how far is the next petrol station, usually displayed in some road signs.
There's an official site for all matters roads, traffic an related laws. It's the General Directorate for Traffic. For now its only available in Spanish.
* Drivers that don't belong to the EU or Switzerland must carry with them the 'Tarjeta Verde' (green card), an insurance that covers expenses of road assitance in case of accident.
All the major car rental companies (Avis, Hertz, Europcar) are available in Spain. Some cities have cheaper options like Easycar in Barcelona (in Barcelona, Madrid, Mallorca and Málaga) and some local car rentals like Pepecar can be a good choice.