What buttons do you push to trade with a villager on minecraft?

Right Click or like placing down a block.

Right click for PC.
For console, whatever the button to place down a block is.

You right-click the villager, and the menu will pop up.
Let's say the trade is a wooden shovel for an emerald.
So you drag the wooden shovel from your inventory, into the box below the trade. Then click your prize.