Welcome to the forums, paoloski!
........And to you, fnord!
As for getting started in airbrushing, nothing beats practice. The best way, to get the feel for the brush, is by spraying on some white paper. You will be able to get a feel for how the paint flows. You want to shoot for a nice pattern or line, with minimal overspray or "spattering" Try this with lager patterns and smaller patterns. Another good way to practice, is on an older model (if you have one) It also give you a chance to test the brush with different air pressure. The best pressure (for me anyway) is between 7-15 PSI. I'm assuming that, you already have a compressor?
Another tip is to make sure that you have a moisture trap installed. All compressors will get droplets of water built up in the tank itself (the nature of compressed air) The trap will catch those droplets before getting to your airbrush and mixing with the paint.
As for mixing paint and thinner, a good rule of thumb is the consistancy of milk. Again, once you start playing around, you will be able to tell if the paint is too thin or too thick.
One major issue, that is often overlooked, is to keep the airbrush clean. Proper cleaning between uses keeps the brush in good working order. If not, you will begin to see the spatter or bad paint flow (not a good thing when painting the model you worked so hard on)
I know it's a lot of information but, just take your time and
experiment. I used to paint strictly from a spray can until I finally purchased an airbrush. I have been using one for over 5 years now and don't know how I managed without it.
The painting forum can answer any other questions you may have.
Good Luck!