Should we take an adeventure to Spain?

I'm 16 and my sister is 18 and we both want to go to Spain during the summer we don't really have a lot money but were going to start saving up. How much should we save up to get a decent hotel and take a train from Valencia to Salamanca? Is it a bad idea going alone without anyone else and just me and my sister? I know a friend who lives in Valencia so I'm not too worried but I'm just looking foward to having a time of my life I love Spaniards any ideas of what we should do there and where should we go? Also we speak spanish....Overall is this a bad ideas what are the pros and cons of going to Spain?