This query recalls the tourist in New York City who asked a street local “How do I get to Carnegie Hall?”. The New Yorker replied “Practice, practice, practice!”. You might enjoy beginning in the Wikipedia with this: “guitar”. To whet your search appetite, a few thoughts.
A standard 6-string guitar is tuned to specific Western Scale notes: E2 A2 D3 G3 B3 and E4. This means that each succeeding string is 5 notes (a Perfect Fourth) above its neighbor. There is one exception to this: B3 is 4 notes (a Major Third) above G3.
There are essentially 12 distinct Key Signatures: C, the six signatures with sharped notes G D A E B F#, and the five signatures with flatted notes Db Ab Eb Bb F. The order of this listing is such that the signature notes are exactly 7 notes (a Perfect Fifth) apart. Thus the relation to the Circle of Fifths diagram. Key signatures have other names (e.g., Gb = F#, Cb = B, C# = Db). When you look at a given sheet music, you see the Key Signature.
To ascertain the Key, the reader must associate the Key Signature and the Mode of the music. There are 7 Modes: Ionian (associated with the Major Key), Dorian, Phyrgian, Lydian, MixoLydian, Aeolian (associated with the Minor Key), and Locrian. Often, the mode is correlated to the first note appearing in the composition.
Example: Key Signature C in Ionian Mode = the C Major Key; Key Signature C in Aeolian Mode = the A Minor Key. Thus, there are 84 (12 signatures * 7 modes) distinct Keys in the Western system.
Two familiar chords (specifically, Triads in the Key Signature C) may tempt further searches: The Major Triad C E G; The Minor Triad A C E. Finally, look at wiki chords. Likely, yau already use thtwo aforementioned triads. Now, continue with the diminished and augmented chords. Good hunting!