I think that a certain aspect of the SOP is not necessarily to detail in specific terms what you hope to expect in research, but more so a narrative of your own preparation and what tools your background offers you to be able to do well in a program.
My undergraduate training is minimal compared to some of my colleagues at other schools (intro + 1 course in each subfield, and I haven't gotten to OT or Minimalism in any course), but I think, as LinguisticMystic mentions that, preparation outside of linguistics can be made relevant to your application.
Going to conferences are a really good idea, it was there that I was able to find out more of what I wanted to do and also got to see what kind of problems are being worked on by professors.