Learning The G Chord
Let's break down and practice the full version of the G major chord. Locate the 3rd fret of the E string and place your middle finger in that fret. Locate the 2nd fret of the A-string and place your index finger in that fret. Finally locate the 3rd fret of the high E string and play this fret with your ring finger.
This is one of the most common ways to play this chord, but it can be pretty tough if your hands are small, so one alternative you can try if that's the case is to replace your ring finger with your pinky.
There's no difference in the sound between these two options, so it's only a matter of which one feels the best for your fingers.
Once you've gotten your fingers in place, let's try picking each of the strings individually or a few at a time. Make sure they're all ringing, and if they're not it's most likely because one of your fretting fingers is muting an adjacent string.
Keep your finger tips up straight, and remember that you can try moving your thumb up and down the back of the neck to see if that helps.