The bottom of your game screen is where you'll find your hot bars, which is where you can store your actions for easier use. The hot bar has 10 slots that you can use at one time, which can sometimes be a pain when you have more than ten items that you want to put on the bar. To place an item on the hot bar, simply open up your skill icon and then left click and hold the skill or combo that you want to put on the hot bar and drag it to the desired place.
Above the hot bar, you'll see a line of text and a bar below it. This is when you are looking at or talking to a mob or NPC. For example, you simply have to target the mob and you will see their health points on the bar. This is a great way to find out if you should attack, or run like everything to get out of there.
To the left of this bar, you'll notice that there is a circle icon that will pop up every now and then. This will let you do certain actions. Normally you'll see this when you have targeted another player, and the icon will give you a couple of different options, such as trading, asking them to join a party, or chatting.
On the very bottom of the game play screen, you'll see a yellow bar. This is your xp bar, or experience bar, and it will tell you how far you have to go to get to the next level in the game. You can also mouse over this bar and see just how much experience you currently have and how much you have to go to level in numbers above the bar.