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 P3D, X-Plane, Microsoft Flight Simulator (2020), FSX and FS9 across several operating systems. 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.

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.

Which pilot client should I use?

Please note that all pilot clients work in Windows. Swift and xSquawkbox also work in Mac OS and Linux.

   My Flight Simulator       Recommended pilot client      Audio for VATSIM Capability     Other compatible clients      Audio for VATSIM Capability 
P3D vPilot In built Swift In built
X-Plane xPilot In built XSquawkbox/Swift Standalone Client Required/In built
Microsoft Flight Simulator (2020) vPilot In built None N/A
FSX vPilot In built Swift In built
FS9 Swift In built None  

Audio for VATSIM Compatibility

In-built - Audio for VATSIM is already built into the client and you need only set up your microphone and speakers/headset and your client connects to Audio for VATSIM.

Standalone Client Required - Audio for VATSIM has not yet been integrated into the client so the standalone Audio for VATSIM client is required. Connect to VATSIM first through your preferred pilot client and then connect to the standalone Audio for VATSIM client.

vPilot - (P3D, Microsoft Flight Simulator 2020 and FSX)

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 is supported by not only the community but by the developer who continues to make feature improvements, and is the most commonly used pilot client on VATSIM.
vPilot is probably the best choice for new members that use P3D, Microsoft Flight Simulator 2020 and FSX because it is quick and easy to install and get started with.

xPilot - (X-Plane only)

xPilot is the newest pilot client built especially for X-Plane but with the ever familiar look and feel of vPilot (for P3D and FSX).

xPilot is probably the best choice for new members that use X-Plane because it is quick and easy to install and get started with.

Swift - (P3D, X-Plane, FSX, FS9)

Swift is a new pilot client for VATSIM currently undergoing open beta testing. It is compatible with all the main pilot clients and several operating systems. It is a feature-rich client but the instructions must be read

xSquawkbox - (X-Plane only)

xSquawkbox is a well established pilot client for connecting to X-Plane and can run on Windows, Mac OS and Linux. The download includes installation instructions.

There is a very useful FAQ here.

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