If Spain is your country of residence then that will go on the pass. Where you start your Interrail trip is not relevant (and neither is your nationality).

If Interrail is worth it depends on what kind of journeys you expect to make. Make a rough plan, look what regular tickets would cost and make the calculation. Do note that some reservations are mandatory and cost extra on top of Interrail (depends a lot on the country).

You have to choose the country where you are from. This means Spain if you live there. Origin → not where you start the journey, but where you live normally!

Please note, that you can use only the Interrail Global Pass 1 time to leave your country or get to the airport and one day to get back. 

Almost all trains in Europe are part of it, but in some countries like France, Spain, Italy, Poland or Sweden you need for all long distance trains reservations. Some other countries like Belgium or The Netherlands only for high speed trains (except ICE) and like in Germany, Switzerland, Austria or Czech Republic just jump into the next train (incl. high speed trains like ICE, Railjet or EC/ECE)

You can get the reservations online on interrail.eu and online at the national carrier (not for ES, FR, IT) or at train stations. For TGV/AVE France-Spain only Paper reservations available → no online ticket. 

You can buy the Interrail Mobile Pass online at interrail.eu or you can get a Paper Pass at every international ticket office (like SNCF in France)