Its like an ebay feeback you get after a trade has taken place. And if you want to increase it, you have to go to the selling/ buying forum and buy or sell some of your stuff.
Welcome to DP. We have a BST (buy sell trade) section in DP. You can do trading there. iTrade is feedback given by the other party (with whom you trade) after the deal is completed. The iTrade is only given when deal is done through a particular DP thread. You can read more about it here. I also advice you to read FAQ and all sticky threads before you you proceed further. DP is very clean place , plz keep it clean