To select a villager, stand in front of them, and place the reticle on them.  Right-click or press the left trigger button on the controller.  This displays a window. The spaces at the top of the window display what the villager has for sale.  The box in the lower-left corner of the window displays what the villager requires for the trade.  You must have the item they want in your inventory to make a trade. To select an item, click it or press the confirm button on your controller.  The item will you are trading will automatically be removed from your inventory, and the item you are purchasing will be placed in your inventory. Villagers have only one or two items when you first trade with them.  The more you trade with them, the more items they will have for sale.

TL;DR in Chinese:
选择一个村民。 查看村民的物品栏。 选择你想买的物品。