C major is the notes C E G.
in its simplest form, you would have C at the bottom, and E two steps higher, and G two steps higher than that.
but when you play it on the guitar, you will notice that you are usually playing more than just 3 notes.
thats because those notes in any order and with any number of repeated notes are still C major.
CEG, ECG, ECEGCE, EGCCCGEGEGCEGGEGCEGCGE....
all C major.
if C (the root) is the lowest note you play, the chord is said to be in "root position"
if E (the 3rd) is the lowest note you play, the chord is said to be in "1st inversion"
if G (the 5th) is the lowest note you play, the chord is said to be in "2nd inversion"
so all triads can exist in root form and 2 inversions because there are 3 possible notes to have as the lowest. (the order of the other notes doesn't affect the inversion, those are just different "voicings")
and all 7th chords can exist in root form and 3 inversions because you have 4 possible notes to be the lowest.
you might notice that this means some chords have more than one name.
for example: ACEF could either be called Amin(b6) or Fmaj7 in 1st inversion
which name they use usually depends on the context of the chords before or after it.
in pop styles, you will see inversions notated as slash chords, showing the name of the chord, and then the bass note specified after the /.
C maj in 2nd inversion = C/G
Fmaj7 in first inversion = Fmaj7/A
in classical styles, you will see inversions notated as "figured bass", showing the intervals above the bass. like V64 or I6. (we show function as roman numerals, so in C, V64 would be G/D, and I6 would be C/E... but you probably dont need to know all that right now)