Well, surely I've taken over nightshifter post now... :(soooorryyyyyyyyyy NS

Bajesus, it's fine going by yourself, Spain is not such a dangerous place and you will have more time to visit places that way. It's not cheap, but it is not expensive either...At least not as expensive as visiting cities like Paris London NY etc...
The best way to socialise is to know some spanish already who can introduce you to others. You will see that the nightlife is vibrant and very exiting...Haven't you heard of the spanish fiesta? we are always parting the night away...any day of the week. Having tried various places in Spain, for me the best nightlife is in Madrid...A couple of tips: Night life only starts after midnight and it carries up until 6 or 7 am. So if you go out at 10 or 11 pm, you'll be alone in the club! Also, not sure if you drink, but if you do, please be drink responsably, spanish hate people who behave drunkly and lose control. And all those kisses...Every time you are introduced to a girl, you are supposed to "kiss" her both cheeks, but not literally or she will be horrified, instead kiss the air if that makes sense..

Ok.. more tips I can think of...hummm, try to learn some spanish words before you get there or you will find it hard to communicate, we are terrible with english or any other foreign language. Rely on public transport, it is outstanding, especially in Madrid. Visit the museums, churches and parks if you can, you will be amazed.
Accomodation is cheap but prices are climbing. Transport is cheap but prices are climbing. Food can be cheap if you find the right places but prices are climbing. Have fun if you get to go there.