IVAO is a network for virtual on-line flying and air traffic controlling.
For on-line flying you need flight simulator software, like Microsoft Flight Simulator or X-Plane.
On-line controlling uses a radar simulation program, especially developed by IVAO itself for this purpose.
With the help of a small IVAO software tool you can connect your flight simulator software to the IVAO Network (IvAn).
You are free to choose yourself what your role will be at that time, either a virtual pilot or a virtual controller, or maybe an observer to get more used to how things are been done.
As a result of this choise next you decide with what software tool you shall connect:
First of all you have to register for free to become a user of the IVAO Network. Everything here is for free!
Go to the registration page (http:/ivao.aero) and follow the instructions there. (Left side menu: Join IVAO!)
After your registration has been completed successfully, you will receive by email a personal identification number (VID) and a password. Check the mailbox of the address which you used for the registration.
You need this VID and password now to download the IVAO software tools necessary to connect to the IVAO Network. Later you need them for connecting to the Network.
The email you received looks like:
Here is your Member VID, IVAN Password and Website Password you requested:
Use VID and IVAN Password to connect with IvAc, IvAp, etc.
Use VID and Website Password when requested in IVAO Website.
Member VID : 654321
IVAN Password : bLabLabLa
Website Password : BlaBlaBlaBla
Note that there are two different passwords.
The first (IVAN) is to connect to the IVAO Network with the special IVAO software programs (IvAp or IvAc). The flying and controlling part.
The second (website) is for connecting to the IVAO websites where entry is only possible for registered users.
Go to the software download page (www.ivao.aero/softdev) and download what you need.
That is:
For pilot:
IvAp, the pilot client, that connects your flight simulator software to the IVAO Network.
Note: FS9 or FSX or X-Plane !!
or 
For controller:
IvAc, the controller client, that is the radar program used to connect to the IVAO Network as ATC.
Notice that always included is "Teamspeak" software for listening and speaking on the radio. The Voice Client.
This voice client comes automatically with the downloads for pilot (IvAp) or controller (IvAc).
Note: A (simple) headset with microphone you have to provide yourself.
There is more stuff there, but that's for later for yourself to check out.
Notice that included in the pilot client, there is a so-called MTL (Multiplayer Traffic Library). You do need at least part of that library, since it helps to add to your realism experience.
This MTL makes it possible for you to see other traffic around you as they are. That is, a B747 will show as a B747. So, during installation, give this MTL install a chance, as convenient.
Even if you want to only fly as a pilot, it is wise to install IvAc, the controller's software. Use it especially in the beginning to watch (and listen!) to what is happening on the Network at those times you are not flying yourself. It is very helpful to observe and learn more and it is a good additional means to eventually contact people on-line to ask for help. Like the pilot client, it contains chat possibilities.
After you have downloaded what you want for now, install it on your computer and .... have a look at the great manuals that are included in your downloads and that now are installed on your computer.
It is a must to read those manuals ....... yes we know, you hate that reading of those manuals ... but then, you do miss a lot of valuable information. Or, you will learn in the hard way....
But ok, continue at least here in the Academy :)
By the way, IVAO has a great Forum (http://forum.ivao.aero/) where you will find separate topics about the different software, in case you have any questions. Free log-in to the Forum is a separate issue. But, you received a website password already, didn't you? See here above :)
For ATC without doubt start as an observer. Log-in in on Teamspeak and IvAc and watch your favourite airfield to see and learn what and how things are happening there.
For pilots it is highly recommended to start flying off-line with a small airplane, single engine, so as to understand the basics of flying. That is, how the aircraft gets into the air and how you could keep it there, while it listens to your commands as you move the yoke and throttles.
A good help in both cases could be the manual made by the BE division at http://www.ivao.be/start_here/pilotmanual/pilot-manual
It gives the full story, from beginning to ..... that's up to you :)
Now you are set to make your first connect. Continue reading at "The bare basics".
Good luck and .... enjoy IVAO!