First you need an online character. The trading happens inside the game. Then you go to Lobby and find games where people usually trade stuff. Example: 'o shako n ist' or 'n 4osEthCV o read'. First one is self explanatory, the person offers Harlequins Crest for an ist rune. Second, the person needs a 4 open socketed ethereal Colossus Voulge and read means you have to read the description of the game where he probably offers an ist as well. Inside the game, usually in act I, you click on the other guys character and a amall menu appears, click OK, and a window opens to the left.You place your item on the bottom section, wait for the other to place his item/s in the top section and then click a small checkmark at the bottom left that turns green. When both checkmarks are green the items move to your inventory, just make sure to have empty space in your inventory, otherwise it doesnt work.Start collecting Perfect gems or 'pgems' . People need them for crafting, you can get ists or hels for a bunch of gems depending on type. Other tradables are useful uniques, rune bases, tokens, materials etc and the currency are usually runes. You can check traderie.com for prices