The recommended place to begin is with a radio listening to your local traffic through a TNC (hardware or software). If you don't already have a hardware TNC, the software ones actually perform much better and only require a soundcard interface to your radio.
You don't need a passcode unless you're planning on sending traffic to the APRS-IS Internet backbone. You can do radio operations or receive-only Internet without one.
If you decide you want to send to APRS-IS, then, if you don't have a Logbook of the World certificate (so you can use the secure means to log into the backbone), you need to privately email the author of your chosen software and tell them your identity and callsign so they can verify you are a licensed amateur radio operator and then issue you a passcode.
Hope this helps.
Andrew, KA2DDO
author of YAAC
