Hai, just i installed Slackware linux , how can i connect this system to internet? I have BSNL broadband (DSL modem conn to ethernet port)

You have DSL modem no router?
With modem you have to look if your modem is supported by the driver and if the corresponding module is compiled (as module or in the kernel, start "make xconfig" in kernel dir)
If your modem is not supported - think about to buy a DSL router.
KDE should have a tool to setup a DSL modem.

This is DSL modem only connected throuch Ethernet, in this config'n only i need to connect to internet

O.K. a DSL router has a modem, too. So your computer has to do all a DSL router does internal, too.
So you need a kernel with pppoe support and the best the tool pppoeconf and therefore KDE should have a frontend.
Check PPPOE support :
- Start root terminal and goto kernel source directory,
- type in "make xconfig" - linux qt-configuration tool starts
- select "Network device support /
- and then - PPP over Ethernet (EXPERIMENTAL)
should have a point or tick if not you have to compile the kernel new with pppoe support.
- if the others have points, make also a point to "PPP over Ethernet (EXPERIMENTAL)"
- save and exit
- make (the kernel)
- make modules_install
- if you have nvidia graphics driver installed yourself it could be that it is deleted so install it new
- move arch/x86/boot/bzimage to /boot/vm-test or what name you want the new kernel to be
- add a new entrance to /boot/grub/menu.lst for the new kernel or what bootloader however you use. (lilo needs to be re-installed)
-restart your computer and select the new kernel to be loaded.

This is for dialin ppp_over_ethernet.
Its nearly the same like with ppp for a normal modem.
After you setup all with pppoeconf (as root) you should be able to start konqueror or firefox, opera or what ever is you favorite browser and start surfing.