I am a level 25 Subtlety Rogue, and at the moment, my DPS is 39.8/19.5. I was wondering, as Rogues are meant to be good damage dealers, is this about normal for my level? If not, what can I do to increase it? Thanks a lot for answering!

It really doesnt matter at that level, but I'd say you are in a good range for your level. Once you hit level 85, thats when your DPS really matters.

go to worldofwarcraft.wikia.com it have basically has everything you need to know