To fly online with VATSIM, you will need to download and install a software application which connects your Flight Simulator with the VATSIM servers. There are several such applications, and all are free. Clients are available for FSX, FS9, P3D and X-Plane. These applications also enable you to communicate with ATC and other aircraft. To do this, a headset with microphone is very useful, but not required. Text communication is always available. There is extensive information and help for all pilot clients in the VATSIM Forums.

PLEASE NOTE: There are a great many other users, both pilots and ATC, logged on to the VATSIM network at any given time. Before you fly online, you are expected to first become familiar with the various aspects of VATSIM such as:

Currently all Pilot clients are written for Windows except XSquawkbox which is only used for X-Plane and can be used in Windows, Mac OS and Linux.

vPilot

http://vpilot.metacraft.com/

(FSX or P3D only) vPilot is a simple, effective client designed to get new pilots flying quickly with excellent model matching so that you can see other online pilots with the correct aircraft type and livery. Generally, it's 5-10 minutes from download to working, including setting up your model matching.  Documentation is thorough.  vPilot was recently released and is supported by not only the community but by the developer who continues to make feature improvements.
 
vPilot is probably the best choice for new members that use FSX or P3D because it is quick and easy to install and get started with.

Squawkbox

http://www.squawkbox.ca/downloads

(FSX, FS9 (2004), FS2002) Squawkbox is an older, but still effective pilot client. It was the first client to introduce visibility of other online pilots therefore display of other aircraft works differently to vPilot and FSInn. Aircraft equipment suffix codes are hard-coded into the software and don't always meet the new ICAO standard although flight plans can be filed via the VATSIM prefile page to overcome this minor restriction.  The download page includes links to help you install and get online.  Squawkbox is no longer supported by the developer, although community support is available in the VATSIM forums.

FSInn

http://www.pcflyer.net/fsinn_setup.zip

(FSX, FS9 (2004), FS2002) FSInn is also an older, but yet effective pilot client. It has a number of features including a traffic radar screen, a push-back feature and the ability to configure model matching so that you can see other online pilots with the correct aircraft type and livery. Aircraft equipment suffix codes are hard-coded into the software and don't always meet the new ICAO standard although flight plans can be filed via the VATSIM prefile page to overcome this minor restriction. The setup can be a bit tricky, but well documented; all of the information regarding FSinn and FScopilot is contained in a very good user manual. FSInn is no longer supported by the developer, although community support is available in the VATSIM forums.

A quick installation guide can be found at http://flightsim.apollo3.com/docs/fsinn/connectiontovatsim.htm

XSquawkbox

http://www.xsquawkbox.net/xsb/download/

(X-Plane only) XSquawkbox is currently the only client to connect X-Plane with VATSIM. X-Plane is an alternate Flight Simulator product to Microsoft Flight Simulator products and Prepar3D and can run on Windows, Mac OS and Linux. The download includes installation instructions.

There is a very useful FAQ here: http://www.xsquawkbox.net/xsb/faq

Some definitions of terms used above:

FSX - Microsoft Flight Simulator X
FS9 - Microsoft Flight Simulator (also known as FS2004)
FS2002 - Microsoft Flight Simulator 2002
P3D - Prepar3D (pronounced “prepared”) a visual simulation platform that allows users to create training scenarios
Model Matching - refers to the process by which the pilot software selects which 3D model and texture to display while connected to VATSIM