I have an Epiphone ES-339. When I tune it, the strings sound fine, then when begin to play a song, the G string sounds out of tune. I only have an issue with that one string and all of them are brand new. I've done some reading and heard things about the bridge, truss rod and saddle but I'm really not sure what the problem is. Any help is much appreciated, thanks.
Very difficult to diagnose without at least some pics, but it could be the intonation of that G string. If that saddle is not in the right position, that string will always play out of tune - unless it's played open. Try tuning using the 12th fret instead of open.
But, better still, check the 12th fret harmonic against 12th fret fretted note. If they're the same, and the string isn't too high above the fingerboard, it's off to a good luthier. Unless you are bending that string in your playing.
Of course, you have checked that the string is still in tune open after playing??
The "G" string seems to be the first string to go out of tune. All my guitars do it under heavy blues style playing. just continue to stretch out the strings, pull them up from the fretboard, tune, play, repeat. once the string tension calms down you'll have less of a problem holding tune.
Here are some troubleshooting ideas. If the G string sounds out of tune when played open: The string is slipping on the capstan of the tuning machine-check the winding; maybe there is a problem with the wire loop at the ball end of the string (rare, it usually just breaks)- replace the string; worn/broken gear in the tuning machine- time for a new tuning machine; string binding in nut slot or bridge saddle then slipping-lightly lubricate at the binding point.
If the G string sounds out of tune when you're playing chords or fretted notes: as Tim says, check the intonation and shift the bridge saddle; check your frets for 'scalloping'(low spots caused by strings gouging out the top of the frets)- time for a fret dress or re fret; G string saddle set too high (unlikely, unless you've been messing with bridge/saddles); neck twist (unlikely, because the other strings would be affected too); excessive neck bow (unlikely, because other strings would be affected too).