If you want to use a standard microphone, you will plug it into an interface which will then connect to a computer via USB or Firewire. Some microphones can be plugged directly into a USB port. This might be a good choice for someone who will only be doing podcasting or similar, but it will severely limit you in the future if you want to expand your horizons as you will be stuck with just that one microphone.
So, either you and your brother will be recording in separate locations or in the same location at the same time and you need to be on two different tracks, right? You can still get two great mics for $500 US in the US. But your prices in Europe for those same mics might be different. My recommendation would be any good dynamic microphone, there are zillions of choices in that category. The $25 Behringer XM8500 has been reported to work very well for voice, the Shure SM58 and beta58 (or similar from other manufacturers) are good options, higher end dynamics that are reported to work well for aggressive rap styles like the Shure SM7b, Electro Voice RE20/RE27/RE320, or Sennheiser MD421 will run upwards of $300 each.
Any info you get from me is pretty much just my uneducated, inexperienced, arrogant opinion. No one has experience with every microphone on the planet. Some have come close.
Look through the threads here in the microphone sections and read as much as you can.
