I agree with everything Gianna'sPapa said. But play around with whatever lens you already have, and decide on what you're missing with that setup; whether it's a wider lens, or more zoom, or whatever. And although the 50mm ($99 version) is highly recommended, it might not fit your style. I haven't used it, but I have a Sigma 30mm; and I after using that, I know I'd be unhappy being constrained to that 50mm range (I'd like a prime a little wider actually). But then again, that's just my personal preference.
As far as the low light ability, I find that the natural 3200 range is about as far as you'd want to go. Anything more might create too much noise for your liking. But if you like to shoot a lot in low light places, definitely consider getting some type of prime lens. I used to think I could manage with not having one. But now that I have one, I just love having that option in my bag.