I've been flight Simming for years and love the aspect of the Virtual Airline. It can be very addicting and once you get your wings under you, you'll be wanting to go to a more serious airline and get that realism.
When I started flying for a virtual airline I started with this airline back on FS2002
They have grown a lot sense I first started, but they are big on fun and short hops rather than long haul routes. You can fly everything from a small Caravan in the back woods of Alaska delivering mail or a 757 out of Dallas bound for Seattle. Its up to you. They do NOT have a very active forum, and you may feel like you're just on your own.
If you're looking for more structure I'd suggest this airline, which is very modern, but easy to get a long with.
This Airline has a training program that you can start flying short hops in a Cessna and then work your way up to a small turbo prop, and so on. They have a route system and you're only able to fly routes in the system as well as aircraft that you are qualified on. They have a lot of cool tools you can download, to make the experience more realistic. They have a very active forum and are very welcoming to do pilots.
My suggestion would be to look into each airline, and make sure you're aware of there requirements. They all come with handbooks and such, and just look at the over all feel of the airline. None of them are bad, some are just better in different ways than others.
I hope this helps,