Expertise in a particular field that you are editing is often a help, as it lets you judge the quality of a site's content with a better understanding of what you are reading. That said, a reasonable understanding of the subject, rather than in depth knowledge is also suitable.
On needs the ability to write clear accurate descriptions of the site in concise terms as well as the ability to judge suitability. Additionaly, one must be able to judge sites fairly on their own merits without undue influence from one's own beliefs/attitudes/prejudices (and we all have them in one form or another).
One also needs to be able to integrate one's efforts into those of the other editors who work with and around you.
My advice would be to find a category of reasonable size (not too large) that holds some interest for you, that you know a bit about, and apply for that. Learn the ropes of writing descriptions, what makes a good addition to a category, what doesn't, when to delete or move a site to somewhere else. There are lots of people you can ask for advice and help as needed.
After getting some experience with this, you can apply for another cat and expand your horizons a bit. Perhaps try joining one of the group efforts and getting experience in more subjects. Eventually, if you stick with it, you will be able to edit effectively, if slowly, in areas in which you have no precise expertise at all.
Good luck!
