The Very Basics and the Main Menu

Ok when you first start the game you'll see that it takes you into a tutorial, it's a bit annoying to listen to Irene talk but make sure you finish these and pay attention, they explain the very basics of the game such as movements, building destruction and the shop. More importantly though, they give you some Entry Change Tickets (the gold envelopes) and a Skill Scroll (the red and gold scroll given by the last tutorial). These items are very important to you as the Entry Change Envelopes are what allow you to change your characters during a battle (as a special note this includes both of the newbie areas as well, meaning Versus Computer and Versus Players) so don't waste them. The skill scroll we will talk about later.
Now that you've gotten out of the tutorial you'll be at the games main lobby screen. This is the hub of the game and where you'll be spending your time while not fighting. At the top is a series of black tabs that say from left to right, Quick Join, Manual Join, Room List, Create Game, Team and Guild, Shop, AP Shop. I will explain each of these tabs in a bit, for now look at the bottom of the screen and in the left you should see a set of semi clear buttons that say, manage hero (where you will be able to see and change your characters equipment once you get some more, see and unlock your characters skills through use of skill scrolls [don't do this yet, we'll do this later), create a hero (where you use hero cards in order to create and use a hero, you should have three trial and two permanent ones left over from the start but don't use them until you've got the concept of the game down), entry setup (where you choose which three heroes, assuming you have more than three available, to bring into a match with you) and tutorial.
Going from right to left they are, quit (which exits the game), options (which lets you configure the game's video and sound to your liking), messenger (which lets you instant message other players in game once you add them to your friends list. This is done by clicking the add friend button [the button that looks like a little man with a plus besides him] and typing their name in), next is the market (where you will buy and sell items once you level up and reach vanguard), beside that is the Website button (which pulls up the games website, which has a great deal of info that could be useful to you and acts as a portal to the forum community, I'd suggest dropping by at least once so you can pick up a few tricks and tips), beside that is the mission board (where you got to look at the mission boards you're working on, mission boards are explained later in more detail) and finally there is the video section (where a variety of useful tutorial vids are stored, take the time to look at these, they could win you a game).
In the middle of the screen is the chat box. This is pretty self explanatory but needless to say you use it to chat with other players.
Finally to the left of that. Is the player status screen and the hero portrait. The hero portrait is where you can choose which of your three heroes is the leader of your three (the one that you start playing as before changing out) by dragging their face over to the space on the far left and dropping it there. Right now your first character is the leader by default.
Now above that is the all important player stat screen. This screen shows your current Win/Loss/Draw total your current ladder points and rank (this is used to determine where you are competetivley and isn't all that important to you right now as a new player). It also shows your nickname and Reputation (which is shown in the form of a happy smiling face). Reputation drops when you leave a game before it is finished and prevents you from getting extra rewards at the end of a game. If you let it drop to low you WILL get kicked out of games a lot, so don't leave just because you're losing.
Finally and most importantly you have your grade. This is represented by both the set of bars in the top corner of the box and the text at the top of it. This ranges from Advanced Militia (the rank you're at now) to Lord of Dimension (the highest rank possible). You advance these ranks by playing and finishing games and accumulating experience (abbreviated exp). Right now it has very little meaning other than you have to get to the green ranks (vanguard) in order to trade in tokens.
Now that we've seen most of the screen, look up to the tabs at the top again. Under these tabs are semi-clear buttons that read, Play With Friends and Novice Map. Take the games advice and choose novice map. In the screen that it pulls up it offers you three options, to play in practice (where you will be alone with the npcs), to play versus the computer (where you will play in a group of three against a computer team) and play vs player (which acts like a shorter version of a standard game).
For now, click play vs computer and get ready for a game.