The wiring in the Bravo (I'm assuming it is a Bravo as you haven't given us any specs to play with here) is standard ISO connectors, so your new HU should plug straight in. You may need an aerial adapter. Some HU's require the constant live to be switched so that settings are retained after you turn the ignition off. This is detailed in the guides already mentioned.
As for a CD changer, well, if you have an Alpine unit, you'll have to make sure any CD changer you buy is compatible with it. Again, ISO connectors are pretty much standard everywhere.
In future, if you need detailed and specific advice, firstly give as much info as possible about the equipment you have and secondly, do look at the guides on the main site as there is probably everything you need to know there.