Hi there,
Welcome to SimV.  I apologize if I don't completely understand your question, but ill do the best to answer your questions.  If your referring to Virtual Airlines and Flying clubs, they usually state if whether they fly online or not.  Some just ask that you to file a report of each flight you did offline (called a PIREP and usually a form on their website)... some ask you to contribute to online ATC sessions.  In any case, the best thing to do is to ask them.

If your just referring to online flight itself, theres a number of places you can goto.  First is zone.com.  Second is a program called FSHostSpy that will help locate online flight sessions that you can fly in.  http://www.chocolatesoftware.com/fshostspy/ ; If you want to get into the really complex stuff and have real live ATC in your flights, id suggest http://www.eskyworld.com/ or http://www.vatsim.com/

I like to fly online personally, everyonce in a while, I will stop by on wspilots's server on FSHost.  Sometimes I like to fly with ATC and I goto esky. 

Anyways, I hope this remotely answer some of the questions you are wondering.

Hey, right now I am flying for Virtual Air Canada which is an amazingly well put together VA in terms of website construction, flyable routes, downloadable aircraft, pilot rankings, as well as online and offline flying. But my best suggestion is check this link out: http://www.avsim.com/links/090_Virtual_Airlines/

...pretty much every VA that is worth checking out is listed there so I am sure you will find something you like...

Once you join a VA getting started is really easy. You will most likely start from the bottom flying small jets or turbo props on regional routes than gradually you will work your way up to flying the heavies. Basically you will get hours by flying your VA's routes and filling out a PIREP which is basically a mini flight log, (i.e. departure and arrival airport, hours flown, distance flown, whether the flight was online or offline, and what equipment you flew the flight in.) That's pretty much it...if you have anymore question Rocket Bird or myself would be happy to answer them.

Hey, keep us updated...did you find a VA that you liked? If not I would highly suggest Virtual Air Canada.