There are many different ways to experience multi-player flight in FSX.
One can join existing sessions via the FSX Multiplayer menu through Gamespy.
One can also join sessions on other servers, one of which the group I fly with uses.
It is over at Digital Theme Park and to get involved there first download and install
Teamspeak3 software for your operating system.
Then go on over to "ts3.digitalthemepark.com" and wait for Zoolander, the
creator and host there to greet you and give you access.
Once in TS3 you can get detailed instructions on what you'll need to do
to join sessions on one of their two servers.
There is a group here that does weekly flights in A2A aircraft and info for that group
can be found in the rather lengthy thread here "Proposal for a multiplayer flight".
VATSIM is another option but I have no knowledge in that realm.
I'm sure more folks will check in here with additional information.
Welcome to the A2A forums and the wonderful Accu-Sim Enhanced FSX world!