@thanaraj.s - I am not sure that using OpenWRT as your main internet router is the best solution for you. It may be easiest and most trouble-free to continue using the equipment provided by your ISP so that you can have a reliable internet connection while you learn about OpenWRT and other networking concepts. This will ensure that you can get tech support from your ISP if/when there is a problem with their equipment, too (they will likely not help you with OpenWRT issues and you could be off-line for a while trying to fix it yourself).
You can, and still should, learn about OpenWRT and networking in general -- you can connect your RPi with OpenWRT as another router behind your main router and play with the configurations until you feel comfortable venturing out on your own (and you can ask specific questions in these forums, too). You should read up on the differences between DSL and Ethernet, how NAT routers work, DNS and the architecture of the internet, and the underlying TCP/IP protocol stack. Maybe even VLANs and VPNs, if you're interested in those, too. It will give you the confidence in the fundamentals so that you can feel comfortable switching over to OpenWRT and troubleshooting your own network.