How To Setup TP-Link Wireless Router Over PTCL Broadband modem.!

Hi please can someone help me of how to setup TP-Link WR642G wireless router on PTCL modem (Broadband)...

And please treat me as a complete beginner because i have seen previous threads, and couldn't understand anything....

I have one of those routers too .. collecting dust .. I bought for cheaps sake .. and now it just wont work .. It give out a signal but no internet .. Life is just difficult like that in the sunny city of karachi ..