The simplest thing
The simplest thing to do at this stage is play two or three chords and keep practisiing a swift change. Here's a good thing to try:
Play a G Major using ONLY the last four strings and your ring finger on the third fret of the high E (last string). Other three strings are played without fretting.
Play C major using ring finger on the second fret of the D string and the index finger on the first fret of the B string. Top 4 strings only again. Don't play the low E and A.
Play D major using index finger on 2nd fret of the G-string (;-)), ring finger on the third fret of the B string and the middle finger on the second fret of the last high E string.
You'll probably get stuck between C and D but a little practice will fix that. Also stand (or sit) relaxed and don't raise your shoulders and don't hold the fretboard as if you're very panicky. That should start you off well. Continue with lessons from websites like www.guitarnoise.com and so.
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