If you don't have a village idol, you have to go to the other guy (no trader does foot deliveries). You'll want to find someone relatively close to you, and for that you first have to find your coordinates on the
world map (you have to look for familiar terrain features to find yourself).
If you do have an idol you can invite the other guy so he can teleport to your idol and you can trade no matter the distance.
There's no trade interface, you exchange objects through a container so trust is needed. Read the Apples for Oranges forum a bit to find traders with lots of positive customer feedback.
As far as currency is concerned, coins exist but nobody uses them. They're sort of useful in vendor stands (automated trade building), but stands aren't very common because they're very vulnerable to theft.
Trade is barter based, and you can pay with all sorts of stuff if you look hard enough. However there are certain goods that are always in high demand and thus serve as a currency of sorts - pearls, floatsams and decent q bear skins. Their value doesn't vary that much between traders, but you should still take some time to read the trade forum and find the best deal for yourself. It's pretty straightforward because most traders post point values on stuff they're buying and selling.