I'll try and cram it all in a few words for you.
In Bridged mode, your PC is directly assigned a public IP address by MTNL.
In PPPoE mode, your modem is assigned a public IP address by MTNL. And in turn, your modem either assigns your PC a
private IP address by DHCP OR you manually assign a private IP to your PC.
If you chose to manually assign a private IP, you should disable DHCP on the modem coz you won't be using it.
Apart from getting an external (public) IP from MTNL, your modem also has an internal (private) IP: 192.168.1.1
Which is also the Default Gateway which you will need in a moment.
So, to answer your questions, you need to assign 192.168.1.x on your PC.
Anything from the range 192.168.1.2 to 192.168.1.254
You first configure the static IP on your PC and THEN on the modem.
If you've got XP... click Start>Control Panel (classic view)>Network Conections>Local Area Connections>right-click>Properties>select Internet Protocol>click Properties
Enter your chosen IP address & Default Gateway.
For
DNS, you can use OpenDNS
208.67.222.222
208.67.220.220
For Host Name and
MAC address
Start>Run>type cmd>and then type ipconfig /all
To get your Host Name and Physical Address (MAC address).
There ya go, short n sweet