Playing posture is the first step in the development of adequate technique. Your sitting position is important too. If you are not currently using a strap while you play I would suggest starting with that to keep the neck of the guitar at a sufficient angle. The next issue is where your thumb of your left hand is on the neck. If you hold the guitar like a baseball bat you'll have a hard time getting to even the 4th fret in the first position. Keep your thumb behind the second finger this should make your hand square to the fretboard. This will make it easier to get to the higher notes.
I have an eBook on my site that is free to download the covers various technique building exercises at http://www.guitarlessonsbuffalony.com

All the best,
J.P.