You will first need to have an APRS-IS server passcode for your callsign, so you can connect to the Internet backbone with permission to transmit packets to the backbone.
There is no way at all to guarantee your Internet packets will get to RF. First off, only text message packets that are addressed to RF stations within range of an I-gate will be forwarded through only the relevant in-range I-gates (plus your next position packet, so the RF recipient knows where the station that sent the text message is located). Secondly, that requires an RF-transmit-capable I-gate within range of your target RF station; there are few of those, due to legal constraints and concerns in different jurisdictions on automatically forwarding packets from the Internet.
There is no way whatsoever to blast your packets to RF over the whole planet. The APRS-IS backbone network and I-gate software are specifically designed to avoid that sort of behavior, since the local RF network is intended for _local_ tactical information (not irrelevant spurious data from out-of-area stations), and the VHF channel doesn't have the bandwidth for the entire planet's worth of APRS stations. If you want to reach the whole planet on RF, you'll need to use the 30-meter HF band on RF yourself (and that has even less total bandwidth than the local VHF channel).
Andrew, KA2DDO
author of YAAC
Hope this helps.
