First of all, you should have friends in Steam
Go to DotA 2 game, in the lower right part, you will see some friends (some of them is online, offline or in DotA2 or maybe other games)
left click one of them (choose one of your friend who also have DotA 2)
there is a selection called "trade items"
and you choose which game items you wanted to trade and just follow trough
Hope you understand this..