you can only really have a nice jam to a song if you care about that song emotionally, for instance, if i blew a metal solo totally (which would be rare, even though im not into metal), it would mean a whole lot less than if i blew a dead solo. I would still feel horrible, but a whole lot less horrible.

Buy a scale book, i always hear people say "learn this first" but really it just depends on your playing style.