The Auction House in Guild Wars 2 works a lot differently than it does in other games. In this game you don't have to go to a specific location to list your auctions and it isn't as easy to understand as it is in most other games. To open the Black Lion Trading Company window (also known as the Auction House window in this game) press the "O" button on your keyboard. This will show you the ingame store where you can buy items with "Gems" if you were to trade your gold for them, as seen on the second tab.
Below the Gems tab you can view the Trading Post Tab. From here you can browse the Auction House for items you might be interested in buying as well as view the Top Valued, Top Supplied, Top Demanded and Top Traded items. Below this tab is the tab I would like to go over a brief bit, the "Sell on Trading Post" Tab. A picture of this tab can be found below so I can explain each element about it better. I had to shrink it down to get it to fit on this page so please click the picture if you're having trouble viewing it.
When you click an item on your list to the left you will get a few different options to your right. This can be a little bit confusing until you get used to the system. The first option you see says "Meet highest buyer" This means that if you select this option the item will sell instantly to the person whom has their auction house set to buy items that are listed at this price. If you want less money but want it instantly this is the way to go. Below that there is an option that says "Match lowest seller".
What this option means is it will list your item up on the Auction House for the same price the lowest one is up for. This means that your item will take longer to sell but you will be getting more money from it when it does.
While this is a pretty simple system to understand but since not many other games work the same way it will confuse a lot of people who are just starting out. On the tab below this tab you will be able to withdrawal any money you have made from past transactions.
That just about covers the Gw2 Auction House and what I found to be the one confusing thing where a lot of new players will have questions. Good luck and happy hunting!