Hi Bernardo -
If you install the full GarageBand library [GarageBand menu > Sound Library > Download All Available Sounds] you should find most of the orchestral instruments in the Library pane [Orchestral > Brass, Strings, Woodwinds, Percussion]. These should be fine for getting started -- later more sophisticated third-party AU orchestral libraries could eventually be purchased and installed..
Here are some links to videos that discuss orchestral composition, orchestration, and creating a virtual orchestra via software instruments. They're not specifically about GarageBand but could serve as an introduction to the concepts and techniques of orchestral writing and virtual orchestration.
These courses cover both orchestral instrument background and more specific MIDI Orchestra topics -- they may be a bit advanced depending on your experience in general with music writing/playing and MIDI/DAW programs:
AFAICT those courses all assume basic musical knowledge. If you need material on formal music theory, here are courses on that, particularly the ones on Music Theory for DAW Musicians:
Again, none of these orchestra-related courses are specific to GarageBand -- serious orchestral work is done in high-end DAWs -- but maybe you can get some familiarity with the process and techniques of virtual orchestration, start experimenting in GB, and then work your way up from there..