The SIM Card can only be used for data, not for calls. The 30 day period starts with first usage, so you can order it in advance.
With this SIM card you can NOT use the "personal Hotspot" functionality many phones offer.
In Spain there are 4 network operators (Movistar, Vodafone, Orange and Yoigo) and a lot more providers. There is a lot of competition with small resellers. In general prepaid SIM cards of the bigger providers are more easy to purchase, but a bit more expensive.