Simple table/chain questions #1-5

[Planet X]

These are extremely basic questions that I wanted to ask in order to gain a
proper
understanding of my firewall and how it works. Please do not email me
telling me
how incredibly stupid I am. I already know that much. ;-)

Examples would be nice too. Any answers will be very gratefully received and
I will
include them in my (very) humble tutorial for newbs.

#1 When a packet comes to my firewall PC from PPP0, what is the first chain
that
      it will enter? PREROUTING?

#2 Depending on whether its a new packet or not, what table does it enter
first?
     I dont understand the roles of NAT, MANGLE, CONNTRACK exactly.

#3 What is the packet comes into my firewall PC from the Eth0? Same thing
     as the PPP0 interface, right?

#4 Do I want to do most of my filtering in PREROUTING and INPUT? If not
    where do I want to do the majority of packet DROPing?

#5 Can I force packets that come in from my eth0 to my firewall PC to go out
     of PPP0 if their destination port is 119 and force packets to go out of
PPP1
     if their destination port is 80?

Thank you very much!

Sincerely,

Joel