They are not expensive, but I don't know it it's worth it. I don't have health problems, just want to lose weight and get as much as exercise as I should. If yes, how many steps should I be taking as a girl in her mid-twenties? Are they clunky and annoying to wear or are there any brands that aren't?

If a pedometer will encourage you to walk more, definitely buy one. They are pretty small and light. 10,000 steps is the recommended amount of steps daily.