Hi Marcelo, and welcome to the hobby!
Yes, as should have been explained during the registration process, you’ll need to take the New Member Orientation Course. That should answer many of your questions, above. You’ll find that at learn.vatsim.net Once you’ve completed that course, you’ll be able to connect to the network.
You may also find the Getting Started checklist right at the top of my.vatsim.net helpful (Getting Started)
We strongly recommend that new users start small. For a new VATSIM member, “start small” means to begin by flying in less busy airspace, choosing familiar airports, and opting for simpler aircraft types, essentially focusing on basic flight operations rather than jumping into complex scenarios or heavily trafficked areas until you gain confidence and experience with the platform. You can find more information on the Pilot Basics page.
You’ll find lots of resources on the VATSIM Pilots page. One of the sections there, Training, points both to the VATSIM Pilot Learning Center as well as the list of VATSIM Training Organizations. The former is a critical reference to help guide you through things like VATSIM Basics, general aviation knowledge and skills, Master Classes, and Frequently Asked Questions. The latter is an optional program where you can learn and earn certifications after demonstrating your piloting skill.
You can do a web search for most common mistakes on VATSIM. There are plenty of them. 
In addition to finding streamers that gear lots of their content to new members, such as Slant Alpha Adventures as just one example, the community often plans events specifically geared toward new members. When checking out the VATSIM Events Page, look specifically for VATSIM First Wings events. These are specifically geared toward new members. You can find the schedule and book a slot on the VATSIM Booking System.
My biggest advice is read, log on and observe, pay attention, and avoid distractions. The more you prepare, the better off you’ll be, but there’s no such thing as starting “100% prepared”. Don’t be afraid to dive in (at a small airfield with ATC staffing), listen, get a decent feel, but then plan and execute a short, easy flight.
Again, welcome!