Dark brown derby is pretty versatile - you can wear it with anything from jeans to chinos to odd trousers ("slacks"), even a suit if you are not English, though I'd prefer an oxford shoe instead. The difference between derby and oxford:
Another thing if you are new: Don't rush into buying things too fast. Read the threads and articles here and elsewhere, get some books (Roetzel's Gentleman, Flusser's dressing the man etc.), but wait a couple of months before you feel confident that you know enough. Your shoes are perfectly serviceable as they are and they should last you long enough before you learn about what makes a good shoe.