Anil,
Welcome to the world of Puppy! I've been using Puppy as my daily working desktop since Dec 2005 and never looked back.
Ok, if you need to configure a BSNL USB modem, here are some steps that should work. Note that Puppy only got the required modules for these modems some time after 2.13 - I think 2.14, maybe? - and I've only used them with 2.17 and 3.00. So better to get one of the latter. These instructions are for 3.00; small change necessary for 2.17, can tell you if need be. These are the instructions that should always work, though not necessarily the fastest.
1. Make sure modem is plugged in and turned on.
2. In the Setup menu in Puppy, open "Network Wizard."
3. You should see a list of interfaces in the "interface" box (if you don't see any, don't worry).
4. Click on "Load module."
5. In the window that opens, click on the "More" tab on top.
6. Click on 'Specify" (it's possible "Auto-probe" would work, but not sure).
7. In the small box that comes up, type "cdc-ether" (without the quotes).
8. It should say "module cdc-ether loaded successfully." Click OK.
9. It should pop a box saying a new interface has been found. Say "Save."
10. When you are back in the main window, under Interfaces, a new button should be present (saying something "eth1" - number will vary). Click it.
11. Choose "Auto DHCP."
12. After a pause, you should get the "Network Configuration Successful" window. Click 'Yes" to save the configuration.
And you're on the Internet. It's not as complicated as it looks, and you only have to do it once - after that the settings will be saved. You may occassionally have to go into Network Wizard and ask it to DHCP again in case you lose the connection.
Finally if you are using a USB cable, a good idea is to switch to an Ethernet cable if you can - the connection is much more reliable and easier to configure both in Puppy and on other operating systems. Your modem probably has an ethernet port - you can get an Ethernet cable (network cable) easily in most cities; a short cable will be less than Rs. 100.