how did you
add[ed] ppp0 gateway and interface
Usually the pppd set's the default route automatically.
Did you use pppconfig to setup the link to your ISP ?
Is your uplink working at all ?
If yes, why do you try to change the default route ?
Which kernel are you running ?
Which services do you want to work ?
http+ftp -> install squid.
"everything" -> compile NAT in you kernel and use ipchains to set it up.
More questions than answers, but your info was simply not enough.