Flying multiplayer flights could not possibly be any easier.
There are several methods, all of which I'll mention, but the last one is my personal favorite (and thus my recommendation to you).
1. The Zone. It sucks, but if you go to
www.zone.com you'll be able to fly online.
2. IP Flights. Using your computer as a host, you tell people your IP (which you can find out from ipchicken.com) and have them enter your IP as the IP to join. Not a bad option if you have a few friends you'd like to fly with.
3. Live ATC. This is, without doubt, the best way to go, IMHO. Nothing beats the reality of talking to an air traffic controller as you fly. Check out my favorite,
www.westcoastatc.com, or take a peek at VATSIM.net, eSkyworld.com or IVAO.org. All decent options.
You don't have to download a program to host a multiplayer session. FSHost will let you do something, but to be honest I've never used it, so I'm not sure what.
If you go with westcoastatc.com, all you'll have to download is TeamSpeak, a voice client which will let you talk to your controllers and vice versa.
Get comfortable!