To find your Steam Trade URL, go to your Steam Profile and select Inventory from the side menu, then select the Trade Offers button. From here you can select the "Who can send me Trade Offers?" option from the Trade menu. Your Trade URL will be listed under the Third-Party Sites section.
Eg. Steam >> Item Inventory >> Trade Offers >> Who can send me Trade Offers?
NOTE: Inventory Privacy must be set to Public to receive Trade Offers.