If you haven't already, sign up for an account.
The Pilot Resource Centre has lots of information available for you to read. If you are new, we advise you have a look at the other pages in the "Basics" section.
In order to connect to the network, you need a piece of free software to connect your flight simulator to our network. You can choose which one you prefer from our * Pilot Software page.
There are lots of other programs which might be useful to you when you are on the network. In order to have a look at who is currently online on the network (pilots and ATC), you can download programs such as VATSpy or vRoute. These are optional, but may help you figure out where you'd like to start.
Load up your flight simulator at a location of your choice. This should be parked on the ground (i.e. on an apron or other parking area). Make sure you aren't blocking the taxiways, on the runway, or in the air.
If you connect to an airport with a Ground (GND) or Tower (TWR) controller, they are likely to be able to give you some help if you need it. For your first few flights, it may be advisable to connect to a smaller airport that isn't very busy, so that controllers will have more time to help you if you need it.
Using the pilot software you installed in step 3a above, connect to VATSIM entering your username, password and callsign. You can select any server you wish - they are all connected to each other. When you have connected you will get a message back from the VATSIM server and may see other pilots at the airport, or controllers in your pilot client's ATC list. You can check to see if there should be someone nearby and check that you're connected to the network using the software in step 3b. (Note: this software only updates who's online every 2 minutes, so don't worry if you don't appear straight away).
How you progress from here is your choice. It might be advisable to tune to a controller's frequency and listen in to get an idea of how things work. You are welcome to connect on the ground at an airport to watch and learn if you aren't ready to make your first flight.
There are lots of articles in the Pilot Resource Centre, especially the General Lessons that can help you get started.
If you are on the network and need help, you can contact a Supervisor. More information about this can be found in the Pilot Resource Centre Basics Section.
Pilot Training Programmes are available should you want someone to help you set up or get started.
The VATSIM Forums and the forums of your local VATSIM sites will likely have plenty of people available to answer questions. Don't be afraid to ask if you're stuck.