Well, the Plaza Mayor is a definite starting point. That's where a lot of the city comes and gathers, especially in the evening and it's extremely easy to meet people by just walking up to them and talking. That's where I met the majority of people I met in Spain.
Specifically, I'm not sure. There are a TON of restaurants, bars, and clubs in Spain so it would be hard to say just where, because pretty much all of them have a great atmosphere. I would recommend doing a bit of bar-hopping and meeting who you can!
Good luck, have fun, and enjoy Spain!